Annual report pursuant to Section 13 and 15(d)

Investments (Details)

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Investments (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Schedule of Investments [Line Items]      
Maturity of investments, maximum 2 years    
Amortized cost $ 144.4 $ 169.7  
Gross unrealized gains 0 0  
Gross unrealized losses 0 (0.1)  
Fair value (net carrying amount) 144.4 169.6  
Available-for-sale debt securities [Abstract]      
Due in one year or less, amortized cost 92.3 76.7  
Due after one year through two years, amortized cost 52.1 93.0  
Total available-for-sale debt securities, amortized cost 144.4 169.7  
Due in one year or less, fair value (net carrying amount) 92.3 76.7  
Due after one year through two years, fair value (net carrying amount) 52.1 92.9  
Total available-for-sale debt securities, fair value (net carrying amount) 144.4 169.6  
Sales of available-for-sale securities 48.9 125.4  
Redemptions of available-for-sale securities 178.8 71.5  
Net adjustment to unrealized investment losses on available-for-sale securities 0.1 0  
Equity contributions to other existing joint ventures     12.4
Dividends from its unconsolidated affiliates 0.3 0.4 2.4
Equity loss 3.8 4.7 3.0
Cummins MerCruiser Diesel Marine LLC joint venture [Member]
     
Available-for-sale debt securities [Abstract]      
Equity loss   3.8  
Agency Bonds [Member]
     
Schedule of Investments [Line Items]      
Amortized cost 59.2 97.7  
Gross unrealized gains 0 0  
Gross unrealized losses 0 (0.1)  
Fair value (net carrying amount) 59.2 97.6  
Corporate Bonds [Member]
     
Schedule of Investments [Line Items]      
Amortized cost 66.4 51.7  
Gross unrealized gains 0 0  
Gross unrealized losses 0 0  
Fair value (net carrying amount) 66.4 51.7  
Commercial Paper [Member]
     
Schedule of Investments [Line Items]      
Amortized cost 16.0 19.5  
Gross unrealized gains 0 0  
Gross unrealized losses 0 0  
Fair value (net carrying amount) 16.0 19.5  
Certificate of Deposit [Member]
     
Schedule of Investments [Line Items]      
Amortized cost 2.0    
Gross unrealized gains 0    
Gross unrealized losses 0    
Fair value (net carrying amount) 2.0    
U.S. Treasury Bills [Member]
     
Schedule of Investments [Line Items]      
Amortized cost 0.8 0.8  
Gross unrealized gains 0 0  
Gross unrealized losses 0 0  
Fair value (net carrying amount) $ 0.8 $ 0.8