Members' Equity (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended			6 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Carlyle/Riverstone Funds	Jul. 31, 2013 Maximum	Sep. 30, 2013 Phantom Units	Sep. 30, 2012 Phantom Units	Mar. 31, 2013 Phantom Units	Sep. 30, 2013 Time-based phantom units	Sep. 30, 2013 Performance-Based phantom units
Members Equity
Common units authorized under DRIP						7,500,000
Common units issued under DRIP					412,717
Cash distribution					$ 5.9
Period over which phantom units vest							3 years
Number of units authorized under the PUPP (in shares)							3,380,474
Weighted average price of granted units (in dollars per share)							$ 12.68	$ 9.99
Number of units available for grant (in shares)							1,724,175		2,045,693
Reconciliation of Phantom Units Outstanding
Balance at the beginning of the period (in shares)							932,007			624,518	307,489
Granted (in shares)							270,042	695,349		135,021	135,021
Vested and paid (in shares)							(122,996)			(61,498)	(61,498)
Forfeited (in shares)							(43,504)			(21,752)	(21,752)
Distribution equivalent rights (in shares)							51,476			33,274	18,202
Balance at the end of the period (in shares)							1,087,025			709,563	377,462
Additional disclosures
Unit-based compensation costs	$ 2.3	$ 2.0	$ 5.6	$ 3.7