Net Income Allocation and Per Unit Data (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended																12 Months Ended
		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2015
Basic and Diluted Earnings Per Limited Partner:
Net income		$ (17,747)		$ (13,332)		$ 39,704		$ 18,275		$ (19,095)		$ (3,238)		$ 55,209		$ 12,058		$ 26,900		$ 44,934		$ 37,556
Less General Partners' interest in net income																		156		252		212
Net income available to limited partners		$ (17,644)		$ (13,253)		$ 39,471		$ 18,170		$ (18,985)		$ (3,219)		$ 54,896		$ 11,990		26,744		44,682		37,344
Less dilutive impact of theoretical distribution of earnings under FASB ASC 260-10-45-60	[1]																	914		4,534		3,318
Limited Partner's interest in net income under FASB ASC 260-10-45-60																		$ 25,830		$ 40,148		$ 34,026
Per unit data:
Basic and diluted net income available to limited partners																		$ 0.48		$ 0.78		$ 0.65
Less dilutive impact of theoretical distribution of earnings under FASB ASC 260-10-45-60	[1]																	0.02		0.08		0.06
Limited Partner's interest in net income under FASB ASC 260-10-45-60		$ (0.32)	[2]	$ (0.24)	[2]	$ 0.59	[2]	$ 0.28	[2]	$ (0.34)	[2]	$ (0.06)	[2]	$ 0.79	[2]	$ 0.19	[2]	$ 0.46	[2],[3]	$ 0.70	[2],[3]	$ 0.59	[3]
Weighted average number of Limited Partner units outstanding																		55,888		57,022		57,285

[1] In any accounting period where the Company's aggregate net income exceeds its aggregate distribution for such period, the Company is required as per FASB ASC 260-10-45-60 to present net income per limited partner unit as if all of the earnings for the period were distributed, based on the terms of the Partnership agreement, regardless of whether those earnings would actually be distributed during a particular period from an economic or practical perspective. This allocation does not impact the Company's overall net income or other financial results.

[2] The sum of the quarters do not add-up to the total due to the weighting of Limited Partner Units outstanding, rounding or the theoretical effects of FASB ASC 260-10-45-60 to Master Limited Partners earnings per unit.

[3] See Note 17 - Earnings Per Limited Partner Units.