Benefit Plans, Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in Plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Amortization of prior service credit (gain) loss	$ (4.7)	$ (5.2)	$ (6.3)
Total recognized in other comprehensive income	8.5	3.2	$ (7.3)
Pension Benefits [Member]
Changes in Plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Current year actuarial gain (loss)	(15.8)	(8.7)
Amortization of actuarial (gain) loss	7.2	6.8
Amortization of prior service credit (gain) loss	0.0	0.4
Total recognized in other comprehensive income	(8.6)	(1.5)
Post-retirement Benefits [Member]
Changes in Plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Current year actuarial gain (loss)	2.6	0.3
Amortization of actuarial (gain) loss	(0.2)	(0.1)
Amortization of prior service credit (gain) loss	(2.3)	(1.9)
Total recognized in other comprehensive income	$ 0.1	$ (1.7)