Earnings Per Limited Partner Unit - Net Income Allocation and Per Unit Data (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended		6 Months Ended
		Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Basic and Diluted Earnings Per Limited Partner:
Net income		$ 58,408	$ 72,325	$ 86,163	$ 74,640
Less General Partner’s interest in net income		409	454	601	469
Net income available to limited partners		57,999	71,871	85,562	74,171
Less dilutive impact of theoretical distribution of earnings under FASB ASC 260-10-45-60		10,283	12,934	14,631	12,294
Limited Partner’s interest in net income under FASB ASC 260-10-45-60		$ 47,716	$ 58,937	$ 70,931	$ 61,877
Per unit data:
Basic and diluted net income available to limited partners		$ 1.25	$ 1.40	$ 1.83	$ 1.42
Less dilutive impact of theoretical distribution of earnings under FASB ASC 260-10-45-60		0.22	0.25	0.31	0.23
Limited Partner’s interest in net income under FASB ASC 260-10-45-60	[1]	$ 1.03	$ 1.15	$ 1.52	$ 1.19
Weighted average number of Limited Partner units outstanding		46,244	51,427	46,760	52,174

[1] See Note 16 - Earnings Per Limited Partner Unit.