Members' Equity (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Mar. 31, 2013
Additional disclosures
Unit-based compensation costs	$ 3.2	$ 1.7
Phantom Units
Members Equity
Period over which 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)	1,769,151		2,045,693
Reconciliation of Phantom Units Outstanding
Balance at the beginning of the period (in shares)	932,007
Granted (in shares)	270,042	695,349
Forfeited (in shares)	(34,434)
Distribution equivalent rights (in shares)	6,500
Balance at the end of the period (in shares)	1,174,115
Time-based phantom units
Reconciliation of Phantom Units Outstanding
Balance at the beginning of the period (in shares)	624,518
Granted (in shares)	135,021
Forfeited (in shares)	(17,217)
Distribution equivalent rights (in shares)	3,250
Balance at the end of the period (in shares)	745,572
Performance-Based phantom units
Reconciliation of Phantom Units Outstanding
Balance at the beginning of the period (in shares)	307,486
Granted (in shares)	135,021
Forfeited (in shares)	(17,217)
Distribution equivalent rights (in shares)	3,250
Balance at the end of the period (in shares)	428,543