Benefit Plans, Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Changes in Plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Amortization of prior service credit (gain) loss	$ (10.0)	$ (7.1)	$ (9.1)
Total recognized in other comprehensive income	18.7	16.1	$ (16.9)
Pension Benefits [Member]
Changes in Plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Current year actuarial gain (loss)	(29.6)	(23.0)
Amortization of actuarial (gain) loss	10.5	7.7
Amortization of prior service credit (gain) loss	0.0	0.0
Total recognized in other comprehensive income	(19.1)	(15.3)
Post-Retirement Benefits [Member]
Changes in Plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Current year actuarial gain (loss)	0.9	(0.2)
Amortization of actuarial (gain) loss	(0.5)	(0.6)
Amortization of prior service credit (gain) loss	0.0	0.0
Total recognized in other comprehensive income	$ 0.4	$ (0.8)