Subsequent Events - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended						12 Months Ended				0 Months Ended	1 Months Ended	0 Months Ended	12 Months Ended	0 Months Ended	12 Months Ended
	Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010	May 14, 2012	Nov. 14, 2012 Subsequent Event	Nov. 07, 2012 Subsequent Event	Nov. 05, 2012 Rangeland Subsequent Event	Sep. 30, 2012 Rangeland Subsequent Event	Nov. 14, 2012 Dividend Paid [Member] Subsequent Event	Sep. 30, 2012 Revolving Credit Facility [Member] Subsequent Event	Sep. 30, 2012 Revolving Credit Facility [Member] Minimum Subsequent Event	Sep. 30, 2012 Revolving Credit Facility [Member] Minimum Junior Subordinated Debt [Member] Subsequent Event
Distribution Made to Member or Limited Partner [Line Items]
Consolidated leverage ratio, maximum							1.00									5.50
Consolidated leverage ratio, minimum							5.00									1.0
Leverage Ratio, Consecutive Quarterly Periods																two consecutive fiscal quarters
Business Acquisition, Cost of Acquired Entity, Purchase Price																	$ 50
Senior Secured Leverage Ratio Maximum																3.75
Senior Secured Leverage Ratio Minimum																1
Debt Instrument, Face Amount																		200
Dividend payable date	Aug. 14, 2012		May 15, 2012		Feb. 14, 2012						Nov. 14, 2012
Initial quarterly distribution per unit	$ 0.38	[1]	$ 0.37	[1]	$ 0.04	[1]	$ 0.37								$ 0.385
Dividend record date	Aug. 07, 2012		May 08, 2012		Feb. 07, 2012							Nov. 07, 2012
Membership interests										100.00%			100.00%
Planned business acquisition cost													425
Revenue								169.9	155.4					193.2
Net income								55.0	44.6					7.4
Partners' Capital Account, Private Placement of Units													$ 225

[1] The $0.04 cash distribution per limited partner unit corresponds to an initial quarterly cash distribution of $0.37 per quarter ($1.48 annually) and represents the prorated distribution for the period of time from December 21, 2011, the closing of the Company's initial public offering, through December 31, 2011, the end of the first fiscal quarter.