Members' Equity - Computation and Distribution Activity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Distributed earnings allocated to:
Total distributed earnings													$ 197.7	$ 187.3	$ 185.5
Undistributed income (loss) allocated to:
Total undistributed loss													(210.9)	25.5	(645.5)
Net income (loss) allocated to:
Total net income (loss)													$ (13.2)	$ 212.8	$ (460.0)
Basic and diluted net income (loss) per unit:
Basic (in dollars per share)	$ (0.34)	$ 0.04	$ 0.15	$ 0.07	$ 1.12	$ 0.03	$ 0.03	$ (0.01)	$ (0.02)	$ 0.00	$ 0.01	$ (2.56)	$ (0.07)	$ 1.18	$ (2.56)
Diluted (in dollars per share)	(0.34)	0.04	0.15	0.07	1.11	0.03	0.03	(0.01)	(0.02)	0.00	0.01	(2.56)	$ (0.07)	$ 1.17	$ (2.56)
Distribution declared/unit (in dollars per share)	$ 0.275	$ 0.271	$ 0.267	$ 0.263	$ 0.259	$ 0.255	$ 0.255	$ 0.255	$ 0.255	$ 0.255	$ 0.255	$ 0.255
Unvested restricted units
Distributed earnings allocated to:
Total distributed earnings													$ 2.8	$ 2.5	$ 2.2
Undistributed income (loss) allocated to:
Total undistributed loss													(3.0)	0.3	(7.5)
Net income (loss) allocated to:
Total net income (loss)													(0.2)	2.8	(5.4)
Common Unit
Distributed earnings allocated to:
Total distributed earnings													194.9	184.8	183.3
Undistributed income (loss) allocated to:
Total undistributed loss													(207.9)	25.2	(638.0)
Net income (loss) allocated to:
Total net income (loss)													$ (13.0)	$ 210.0	$ (454.6)