Annual report pursuant to Section 13 and 15(d)

Debt (Details)

v3.8.0.1
Debt (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Long-term Debt [Abstract]      
Total Long-Term Debt Including Current Maturities $ 437.4 $ 439.4  
Current Maturities of Long-term Debt (5.6) (5.6)  
Long-Term Debt, Net 431.8 433.8  
Maturities of Long-term Debt [Abstract]      
2018 5.6    
2019 5.9    
2020 5.8    
2021 153.3    
2022 0.3    
Thereafter 266.5    
Total Long-Term Debt Including Current Maturities 437.4 439.4  
Line of Credit Facility [Abstract]      
Line of Credit Facility, Maximum Borrowing Capacity 300.0    
Line of Credit Facility, Remaining Borrowing Capacity 295.7    
Letters of Credit Outstanding, Amount $ 5.3    
Minimum [Member]      
Line of Credit Facility [Abstract]      
Debt Instrument, Covenant, Interest Coverage Ratio 3.00    
Maximum [Member]      
Line of Credit Facility [Abstract]      
Debt Instrument, Covenant, Leverage Ratio 3.50    
Letter of Credit [Member]      
Line of Credit Facility [Abstract]      
Letters of Credit Outstanding, Amount $ 4.3    
Line of Credit [Member]      
Line of Credit Facility [Abstract]      
Line of Credit Facility, Increase (Decrease), Other, Net $ 100.0    
Line of Credit Facility, Commitment Fee Percentage 0.20%    
Line of Credit [Member] | Minimum [Member]      
Line of Credit Facility [Abstract]      
Line of Credit Facility, Commitment Fee Percentage 0.125%    
Line of Credit [Member] | Maximum [Member]      
Line of Credit Facility [Abstract]      
Line of Credit Facility, Commitment Fee Percentage 0.35%    
Line of Credit [Member] | London Interbank Offered Rate (LIBOR) [Member]      
Line of Credit Facility [Abstract]      
Line of Credit Facility, Interest Rate During Period 1.30%    
Line of Credit [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]      
Line of Credit Facility [Abstract]      
Line of Credit Facility, Interest Rate During Period 1.00%    
Line of Credit [Member] | London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]      
Line of Credit Facility [Abstract]      
Line of Credit Facility, Interest Rate During Period 1.90%    
Line of Credit [Member] | Base Rate [Member]      
Line of Credit Facility [Abstract]      
Line of Credit Facility, Interest Rate During Period 0.30%    
Line of Credit [Member] | Base Rate [Member] | Minimum [Member]      
Line of Credit Facility [Abstract]      
Line of Credit Facility, Interest Rate During Period 0.00%    
Line of Credit [Member] | Base Rate [Member] | Maximum [Member]      
Line of Credit Facility [Abstract]      
Line of Credit Facility, Interest Rate During Period 0.90%    
Notes Due 2027 [Member]      
Long-term Debt [Abstract]      
Total Long-Term Debt Including Current Maturities $ 162.4 162.3  
Interest Rate 7.125%    
Due date 2027    
Discount on Long-term Debt $ 0.4 0.4  
Debt Issuance Costs, Net 0.4 0.5  
Maturities of Long-term Debt [Abstract]      
Total Long-Term Debt Including Current Maturities 162.4 162.3  
Senior Notes Due 2021 [Member]      
Long-term Debt [Abstract]      
Total Long-Term Debt Including Current Maturities [1] $ 148.2 147.8  
Interest Rate 4.625%    
Due date 2021    
Debt Issuance Costs, Net $ 1.5 1.9  
Maturities of Long-term Debt [Abstract]      
Total Long-Term Debt Including Current Maturities [1] 148.2 147.8  
Debentures Due 2023 [Member]      
Long-term Debt [Abstract]      
Total Long-Term Debt Including Current Maturities [1] $ 103.4 103.4  
Interest Rate 7.375%    
Due date 2023    
Discount on Long-term Debt $ 0.2 0.2  
Debt Issuance Costs, Net 0.2 0.2  
Maturities of Long-term Debt [Abstract]      
Total Long-Term Debt Including Current Maturities [1] 103.4 103.4  
Loan With Fond Du Lac County Economic Development Corporation Due 2021 [Member]      
Long-term Debt [Abstract]      
Total Long-Term Debt Including Current Maturities $ 19.6 23.8  
Interest Rate 2.00%    
Due date 2021    
Discount on Long-term Debt $ 3.0 3.8  
Debt Issuance Costs, Net 0.1 0.1  
Maturities of Long-term Debt [Abstract]      
Total Long-Term Debt Including Current Maturities 19.6 23.8  
Line of Credit Facility [Abstract]      
Total Loan Forgiveness $ 2.1 $ 2.1 $ 2.0
Annual Forgiveness Percent 43.00% 43.00% 41.00%
Loan With Fond Du Lac County Economic Development Corporation Due 2021 [Member] | Maximum [Member]      
Line of Credit Facility [Abstract]      
Annual Forgiveness Percent 43.00%    
Notes Payable Through 2027 [Member]      
Long-term Debt [Abstract]      
Total Long-Term Debt Including Current Maturities $ 3.8 $ 2.1  
Interest Rate 5.892%    
Due date 2027    
Discount on Long-term Debt $ 0.2 0.2  
Maturities of Long-term Debt [Abstract]      
Total Long-Term Debt Including Current Maturities $ 3.8 $ 2.1  
[1] Included in Senior notes, 4.625 percent due 2021 and Debentures, 7.375 percent due 2023 at December 31, 2017 and December 31, 2016, are the aggregate fair values related to the fixed-to-floating interest rate swaps as discussed in Note 14 – Financial Instruments.