Income Taxes - Components of Income Tax Benefit (Provision) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Current income tax expense	$ (0.1)	$ (0.3)
Deferred income tax benefit (expense)	(1.3)	34.0
Income tax benefit (expense)	(1.4)	33.7
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Expected income tax benefit based on federal statutory rate	2.4	67.3
State income tax benefit, net of federal benefit	0.2	8.0
Unit-based compensation	(2.5)	2.4
Non-deductible expense related to goodwill impairment	0.0	(43.4)
Change in valuation allowance	(1.2)	0.0
Other	(0.3)	(0.6)
Income tax benefit (expense)	$ (1.4)	$ 33.7