Accumulated Other Comprehensive Income - Components of Comprehensive Income (Loss), After-tax (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ 2,672,500,000	$ 3,451,600,000	$ 3,655,800,000
Ending Balance	981,300,000	2,672,500,000	3,451,600,000
Operating costs and expenses (exclusive of items shown separately below)	2,524,900,000	3,536,600,000	4,071,400,000
Foreign Currency Translation Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(4,300,000)	(4,500,000)	1,400,000
Current period change	6,100,000	200,000	(5,900,000)
Reclassification from other comprehensive income to earnings	0	0
Ending Balance	1,800,000	(4,300,000)	(4,500,000)
Prior Service Credit (Cost) Associated with Postretirement Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	35,900,000	44,600,000	0
Current period change	185,400,000	0	44,600,000
Reclassification from other comprehensive income to earnings	(17,300,000)	(8,700,000)
Ending Balance	204,000,000.0	35,900,000	44,600,000
Prior Service Credit (Cost) 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)	17,300,000	8,700,000
Total Accumulated Other Comprehensive Income
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	31,600,000	40,100,000	1,400,000
Current period change	191,500,000	200,000	38,700,000
Reclassification from other comprehensive income to earnings	(17,300,000)	(8,700,000)	0
Ending Balance	$ 205,800,000	$ 31,600,000	$ 40,100,000