Pension Plans - Qualified and Non-Qualified Plans - Amounts Recognized in Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax [Abstract]
Unrecognized actuarial losses included in other comprehensive loss, net of tax	$ 2.0	$ 2.2
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Unrecognized actuarial losses included in other comprehensive loss	3.0	3.1
Pension Plan [Member]
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Reclassification Adjustment, before Tax [Abstract]
Net gain (loss) recognized in other comprehensive income	(2.3)	2.1
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax [Abstract]
Unrecognized actuarial losses included in other comprehensive loss, net of tax	30.7	28.4
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Unrecognized actuarial losses included in other comprehensive loss	$ 37.4	$ 35.2