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Retirement and Deferred Compensation Plans - Components of Net Periodic Benefit Cost and Other Amounts Recognized in Other Comprehensive Loss (Income) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension plans      
Net Periodic Benefit Cost      
Service cost $ 10.1 $ 8.0 $ 7.2
Interest cost 10.6 11.8 12.7
Expected return on assets (10.8) (10.9) (12.9)
Curtailment and settlement 0.0 (6.9) 0.0
Net loss 0.9 1.0 4.0
Prior service cost (credit) 0.4 0.4 0.4
Net periodic benefit cost 11.2 3.4 11.4
Other Changes in Plan Assets and Benefit Obligation Recognized in Other Comprehensive Income/Loss      
Net (gain) loss (1.5) 24.7 (22.9)
Prior service cost 2.9 0.0 0.0
Amortization of net loss (0.9) (1.0) (4.0)
Amortization of prior service (cost) credit (0.4) (0.4) (0.4)
Total recognized in other comprehensive loss 0.1 23.3 (27.3)
Total recognized in net periodic benefit cost and other comprehensive income/loss 11.3 26.7 (15.9)
Retiree Health Care Plan      
Net Periodic Benefit Cost      
Interest cost 0.6 0.7 0.7
Net loss 0.1 0.1 0.1
Prior service cost (credit) (0.8) (0.8) (0.8)
Net periodic benefit cost (0.1) 0.0 0.0
Other Changes in Plan Assets and Benefit Obligation Recognized in Other Comprehensive Income/Loss      
Net (gain) loss (0.3) 0.2 (0.5)
Amortization of net loss (0.1) (0.1) (0.1)
Amortization of prior service (cost) credit 0.8 0.8 0.8
Total recognized in other comprehensive loss 0.4 0.9 0.2
Total recognized in net periodic benefit cost and other comprehensive income/loss $ 0.3 $ 0.9 $ 0.2