Accumulated Other Comprehensive Income - Components of Comprehensive Income (Loss), After-tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ 981.3	$ 2,672.5	$ 3,451.6
Ending Balance	1,820.8	981.3	2,672.5
Operating costs and expenses (exclusive of items shown separately below)	2,553.1	2,524.9	3,536.6
Foreign Currency Translation Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	1.8	(4.3)	(4.5)
Current period change	(1.0)	6.1	0.2
Reclassification from other comprehensive income to earnings	0.0	0.0	0.0
Ending Balance	0.8	1.8	(4.3)
Prior Service Credit Associated with Postretirement Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	204.0	35.9	44.6
Current period change	139.5	185.4	0.0
Reclassification from other comprehensive income to earnings	(46.4)	(17.3)	(8.7)
Ending Balance	297.1	204.0	35.9
Prior Service Credit Associated with Postretirement Plans | Reclassification out of Accumulated Other Comprehensive Income | Postretirement health care and life insurance benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Operating costs and expenses (exclusive of items shown separately below)	46.4	17.3	8.7
Total Accumulated Other Comprehensive Income
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	205.8	31.6	40.1
Current period change	138.5	191.5	0.2
Reclassification from other comprehensive income to earnings	(46.4)	(17.3)	(8.7)
Ending Balance	$ 297.9	$ 205.8	$ 31.6