Income Taxes - Book Income Reconciliation To Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Expected income tax benefit (expense) based on federal statutory tax rate	$ 58.5	$ 233.6	$ (1.0)
State income tax benefit (expense), net of federal benefit	6.5	27.0	(0.1)
Unit-based compensation	(6.0)	(2.2)	(0.7)
Non-deductible expense related to impairments	(43.4)	(264.5)	(10.7)
Change in valuation allowance	153.3	0.0	0.0
Other	(5.5)	(0.8)	(5.7)
Total income tax expense	$ (143.2)	$ (6.9)	$ (18.2)