Annual report pursuant to Section 13 and 15(d)

Postretirement Benefits Pretax Activity in Accumulated Other Comprehensive Income (Loss) (Details)

v3.8.0.1
Postretirement Benefits Pretax Activity in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Prior Service Costs (Credits) [Abstract]      
Amount Recognized as Component of Net Benefit Costs $ (0.7) $ (0.7) $ (1.3)
Net Actuarial Losses [Abstract]      
Amount Recognized as Component of Net Benefit Costs $ 111.8 73.1 103.8
Percentage of Benefit Obligation or Market Value of Assets, Over Which Actuarial Gains and Losses are Subject to Amortization (in Hundredths) 10.00%    
Pension Plans [Member]      
Prior Service Costs (Credits) [Abstract]      
Beginning Balance $ 0.0 0.0  
Amount Recognized as Component of Net Benefit Costs 0.0 0.0  
Ending Balance 0.0 0.0 0.0
Net Actuarial Losses [Abstract]      
Beginning Balance 397.1 457.8  
Actuarial (Gains) Losses Arising During the Period 16.0 11.8  
Amount Recognized as Component of Net Benefit Costs (111.0) (72.5)  
Ending Balance 302.1 397.1 457.8
Total 302.1 397.1  
Estimated Pretax Net Actuarial Loss in Accumulated Other Comprehensive Income (Loss) Expected to be Recognized Over the Next Year 9.9    
Other Postretirement Benefit Plans [Member]      
Prior Service Costs (Credits) [Abstract]      
Beginning Balance (10.2) (10.9)  
Amount Recognized as Component of Net Benefit Costs 0.7 0.7  
Ending Balance (9.5) (10.2) (10.9)
Net Actuarial Losses [Abstract]      
Beginning Balance 1.0 1.9  
Actuarial (Gains) Losses Arising During the Period 0.0 (0.9)  
Amount Recognized as Component of Net Benefit Costs 0.0 0.0  
Ending Balance 1.0 1.0 $ 1.9
Total (8.5) $ (9.2)  
Estimated Pretax Net Actuarial Loss in Accumulated Other Comprehensive Income (Loss) Expected to be Recognized Over the Next Year 0.0    
Estimated Pretax Prior Service (Cost) Credit in Accumulated Other Comprehensive Income (Loss) Expected to be Recognized Over the Next Year $ 0.7