Equity (Details) (USD $)	0 Months Ended	3 Months Ended								6 Months Ended	12 Months Ended
	May 11, 2011	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Mar. 31, 2013	Mar. 31, 2012
Partnership equity
General partner, interest (as a percent)											0.10%	0.10%
Limited partners, interest (as a percent)											99.90%	99.90%
Limited Partner Equity
Units										10,933,568
Amount										$ 47,225,000
Initial Public Offering
Proceeds from sale of common units, net of offering costs											(642,000)	74,759,000
Units outstanding after completion of IPO										10,933,568
Unaudited Pro Forma
Limited Partner Equity
Units										10,933,568
Amount										47,225,000
Initial Public Offering
Units outstanding after completion of IPO										10,933,568
Common units
Limited Partner Equity
Units	8,864,222	47,703,313				23,296,253				10,933,568	47,703,313	23,296,253
Amount		920,998,000				384,604,000				47,225,000	920,998,000	384,604,000
Earnings per unit, basic and diluted
BASIC AND DILUTED EARNINGS PER LIMITED PARTNER UNIT: (in dollars per unit)		$ 0.39	$ 0.75	$ 0.18	$ (0.76)	$ 0.47	$ 0.24	$ (0.36)	$ (0.53)	$ 1.16	$ 0.96	$ 0.32
Initial Public Offering
Number of units sold in initial public offering	4,025,000
Price per unit of units sold in initial public offering (in dollars per share)	$ 21.00
Number of units repurchased	175,000
Units outstanding after completion of IPO	8,864,222	47,703,313				23,296,253				10,933,568	47,703,313	23,296,253
Common units | Repayment of debt
Initial Public Offering
Number of units sold in initial public offering	3,850,000
Proceeds from sale of common units, net of offering costs	71,900,000
Offerings costs	9,000,000
Common units | Purchase of common units
Initial Public Offering
Number of units sold in initial public offering	175,000
Proceeds from sale of common units, net of offering costs	3,400,000
Common units | Unaudited Pro Forma
Limited Partner Equity
Units										5,014,222
Amount										21,658,000
Earnings per unit, basic and diluted
BASIC AND DILUTED EARNINGS PER LIMITED PARTNER UNIT: (in dollars per unit)										$ 1.16
Initial Public Offering
Units outstanding after completion of IPO										5,014,222
Subordinated units
Partnership Equity
Number of consecutive, non-overlapping four-quarter periods to determine termination of subordination period											3
Number of non-overlapping quarters											4
Number of quarters during which quarterly distributions are earned or paid to determine termination of subordination period											4
Subordinated units, conversion ratio											1
Common units converted to subordinated units	5,919,346
Limited Partner Equity
Units	5,919,346	5,919,346				5,919,346					5,919,346	5,919,346
Amount		13,153,000				19,824,000					13,153,000	19,824,000
Earnings per unit, basic and diluted
BASIC AND DILUTED EARNINGS PER LIMITED PARTNER UNIT: (in dollars per unit)		$ 0.39	$ 0.75	$ 0.18	$ (0.77)	$ 0.53	$ 0.28	$ (0.36)	$ (0.53)		$ 0.93	$ 0.58
Initial Public Offering
Units outstanding after completion of IPO	5,919,346	5,919,346				5,919,346					5,919,346	5,919,346
Subordinated units | Minimum
Partnership Equity
Percentage of quarterly distribution earned and paid to determine termination of subordination period											150.00%
Subordinated units | Unaudited Pro Forma
Limited Partner Equity
Units										5,919,346
Amount										$ 25,567,000
Earnings per unit, basic and diluted
BASIC AND DILUTED EARNINGS PER LIMITED PARTNER UNIT: (in dollars per unit)										$ 1.16
Initial Public Offering
Units outstanding after completion of IPO										5,919,346