Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2021
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period		$ 981.3
Balance, end of period	$ 951.8	951.8
Foreign Currency Translation Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period		1.8
Reclassification from other comprehensive income to earnings		0.0
Current period change		(0.4)
Balance, end of period	1.4	1.4
Prior Service Credit Associated with Postretirement Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period		204.0
Reclassification from other comprehensive income to earnings	(22.0)
Current period change		0.0
Balance, end of period	182.0	182.0
Accumulated other comprehensive income
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period	194.6	205.8
Reclassification from other comprehensive income to earnings		(22.0)
Current period change		(0.4)
Balance, end of period	$ 183.4	$ 183.4