Members' Equity - Computation and Distribution Activity (Details) - USD ($) $ / shares in Units, $ in Millions							3 Months Ended								9 Months Ended
	Nov. 13, 2020	Aug. 13, 2020	May 13, 2020	Nov. 13, 2019	Aug. 13, 2019	May 14, 2019	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019
Distributed earnings allocated to:
Total distributed earnings							$ 46.6				$ 139.6				$ 139.9	$ 389.8
Undistributed loss allocated to:
Total undistributed loss							(34.0)				(127.8)				(410.0)	(570.4)
Net income (loss) allocated to:
Total net income (loss)							$ 12.6				$ 11.8				$ (270.1)	$ (180.6)
Basic and diluted net income (loss) per unit:
Basic (in dollars per share)							$ 0.03				$ 0.02				$ (0.55)	$ (0.40)
Diluted (in dollars per share)							0.03				0.02				$ (0.55)	$ (0.40)
Distribution declared/unit (in dollars per share)							$ 0.09375	$ 0.09375	$ 0.09375	$ 0.1875	$ 0.283	$ 0.283	$ 0.279	$ 0.275
Distribution paid per unit (in dollars per share)		$ 0.09375	$ 0.09375	$ 0.283	$ 0.283	$ 0.279
Subsequent Event
Basic and diluted net income (loss) per unit:
Distribution declared/unit (in dollars per share)	$ 0.09375
Distribution paid per unit (in dollars per share)	$ 0.09375
Unvested restricted units
Distributed earnings allocated to:
Total distributed earnings							$ 0.7				$ 1.6				$ 2.3	$ 4.6
Undistributed loss allocated to:
Total undistributed loss							(0.6)				(1.5)				(7.0)	(6.8)
Net income (loss) allocated to:
Total net income (loss)							0.1				0.1				(4.7)	(2.2)
Common units
Distributed earnings allocated to:
Total distributed earnings							45.9				138.0				137.6	385.2
Undistributed loss allocated to:
Total undistributed loss							(33.4)				(126.3)				(403.0)	(563.6)
Net income (loss) allocated to:
Total net income (loss)							$ 12.5				$ 11.7				$ (265.4)	$ (178.4)