Members' Equity - Computation and Distribution Activity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Distributed earnings allocated to:
Total distributed earnings	$ 47.0		$ 46.6
Undistributed loss allocated to:
Total undistributed loss	(59.7)		(333.4)
Net loss allocated to:
Total net loss	$ (12.7)		$ (286.8)
Basic and diluted total net loss per unit:
Basic (in dollars per share)	$ (0.03)		$ (0.59)
Diluted (in dollars per share)	(0.03)		(0.59)
Distribution declared/unit (in dollars per share)	$ 0.09375	$ 0.09375	$ 0.09375	$ 0.1875
Unvested restricted units
Distributed earnings allocated to:
Total distributed earnings	$ 1.1		$ 0.8
Undistributed loss allocated to:
Total undistributed loss	(1.4)		(6.0)
Net loss allocated to:
Total net loss	(0.3)		(5.2)
Common units
Distributed earnings allocated to:
Total distributed earnings	45.9		45.8
Undistributed loss allocated to:
Total undistributed loss	(58.3)		(327.4)
Net loss allocated to:
Total net loss	$ (12.4)		$ (281.6)