Earnings per Limited Partner Units - 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, 2020		Jun. 30, 2020	Mar. 31, 2020		Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2018
Basic and Diluted Earnings Per Limited Partner:
Net income		$ (30,199)		$ (46)	$ 58,408		$ 27,755		$ (33,905)		$ (23,098)		$ 72,325		$ 2,315		$ 55,918		$ 17,637		$ 55,505
Less General Partners’ interest in net income																	377		95		314
Net income available to limited partners		$ (29,976)		$ (45)	$ 57,999		$ 27,563		$ (33,681)		$ (22,948)		$ 71,871		$ 2,300		55,541		17,542		55,191
Less dilutive impact of theoretical distribution of earnings	[1]																6,812				6,340
Limited Partner’s interest in net income																	$ 48,729		$ 17,542		$ 48,851
Per unit data:
Basic and diluted net income available to limited partners																	$ 1.22		$ 0.35		$ 1.01
Less dilutive impact of theoretical distribution of earnings	[1]																0.15				0.12
Limited Partner’s interest in net income under		$ (0.68)	[2]		$ 1.03	[2]	$ 0.49	[2]	$ (0.69)	[2]	$ (0.46)	[2]	$ 1.15	[2]	$ 0.04	[2]	$ 1.07	[2],[3]	$ 0.35	[2],[3]	$ 0.89	[3]
Weighted average number of Limited Partner units outstanding																	45,656		50,814		54,764

[1] In any accounting period where the Company’s aggregate net income exceeds its aggregate distribution for such period, the Company is required 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 19 - Earnings Per Limited Partner Units.