Income Taxes - Components of Income Tax Benefit (Provision) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Current income tax expense	$ (0.4)	$ (0.7)	$ (1.1)	$ (2.0)
Deferred income tax benefit (expense)	(5.6)	(5.6)	17.1	(0.7)
Income tax benefit (expense)	(6.0)	(6.3)	16.0	(2.7)
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Expected income tax benefit (expense) based on federal statutory rate	(3.6)	(3.8)	60.0	37.4
State income tax benefit (expense), net of federal benefit	(0.6)	(0.7)	6.4	3.6
Unit-based compensation	(1.6)	(1.3)	(6.0)	(1.3)
Non-deductible expense related to goodwill impairment	0.0	0.0	(43.4)	(43.8)
Other	(0.2)	(0.5)	(1.0)	1.4
Income tax benefit (expense)	$ (6.0)	$ (6.3)	$ 16.0	$ (2.7)