Income Taxes - Components of Income Tax Benefit (Provision) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Current income tax expense	$ (0.1)	$ (0.4)	$ (0.2)	$ (1.1)
Deferred income tax benefit (expense)	(4.3)	(5.6)	(12.2)	17.1
Income tax benefit (expense)	(4.4)	(6.0)	(12.4)	16.0
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Expected income tax benefit (expense) based on federal statutory rate	(2.0)	(3.6)	4.2	60.0
State income tax benefit (expense), net of federal benefit	(0.3)	(0.6)	0.5	6.4
Unit-based compensation	(0.2)	(1.6)	(3.1)	(6.0)
Non-deductible expense related to goodwill impairment	0.0	0.0	0.0	(43.4)
Change in valuation allowance	(1.6)	0.0	(3.8)	0.0
Oklahoma statutory rate change	0.0	0.0	(7.6)	0.0
Other	(0.3)	(0.2)	(2.6)	(1.0)
Income tax benefit (expense)	$ (4.4)	$ (6.0)	(12.4)	$ 16.0
Deferred state income tax expense (benefit)			$ 7.6