Annual report pursuant to Section 13 and 15(d)

Postretirement Benefits Fair Values of Trust's Pension Assets (Details)

v3.8.0.1
Postretirement Benefits Fair Values of Trust's Pension Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets $ 546.1 [1] $ 687.9 [2]
Fair Value, Inputs, Level 1 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 15.2 [1] 1.5 [2]
Fair Value, Inputs, Level 2 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 447.3 [1] 532.5 [2]
Short-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 15.2 1.9
Short-Term Investments [Member] | Fair Value, Inputs, Level 2 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 0.0 0.0
Government Fixed-Income Securities [Member] | Fair Value, Inputs, Level 1 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 0.0 0.0
Government Fixed-Income Securities [Member] | Fair Value, Inputs, Level 2 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 114.1 141.5
Corporate Fixed-Income Securities [Member] | Fair Value, Inputs, Level 1 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 0.0 0.0
Corporate Fixed-Income Securities [Member] | Fair Value, Inputs, Level 2 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 324.0 382.1
Other Investment Companies [Member] | Fair Value, Inputs, Level 1 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 0.0 (0.4)
Other Investment Companies [Member] | Fair Value, Inputs, Level 2 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 9.2 8.9
Other Pension Plan Liabilities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets   (58.6)
Estimate of Fair Value Measurement [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 462.5 [1] 534.0 [2]
Estimate of Fair Value Measurement [Member] | Short-Term Investments [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 15.2 1.9
Estimate of Fair Value Measurement [Member] | Government Fixed-Income Securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 114.1 141.5
Estimate of Fair Value Measurement [Member] | Corporate Fixed-Income Securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 324.0 382.1
Estimate of Fair Value Measurement [Member] | Other Investment Companies [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 9.2 8.5
Estimate of Fair Value Measurement [Member] | Other Pension Plan Liabilities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets (36.1)  
Portion at Other than Fair Value Measurement [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 119.7 [1] 212.5 [2]
Portion at Other than Fair Value Measurement [Member] | Short-Term Investments [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 25.0 46.8
Portion at Other than Fair Value Measurement [Member] | Equity Securities [Member] | United States [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 23.9 132.8
Portion at Other than Fair Value Measurement [Member] | Equity Securities [Member] | International [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets 5.6 17.6
Portion at Other than Fair Value Measurement [Member] | Commingled Funds [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total Pension Plan Net Assets $ 65.2 $ 15.3
[1] (A)See Note 7 – Fair Value Measurements for a description of levels within the fair value hierarchy. The level in the fair value hierarchy within which the fair value measurement is classified is determined based on the lowest level input that is significant to the fair value measurement in its entirety. A description of the valuation methodologies is provided following these tables. There were no transfers in and/or out of Level 1 and Level 2 in 2017.
[2] (A)See Note 7 – Fair Value Measurements for a description of levels within the fair value hierarchy. The level in the fair value hierarchy within which the fair value measurement is classified is determined based on the lowest level input that is significant to the fair value measurement in its entirety. A description of the valuation methodologies is provided following these tables. There were no transfers in and/or out of Level 1 and Level 2 in 2016.