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, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2019	Mar. 31, 2018
Basic and Diluted Earnings Per Limited Partner:
Net income		$ 72,325	$ 2,315	$ 54,778	$ 30,182	$ 74,640	$ 84,960
Less General Partner’s interest in net income		454		319		469	494
Limited Partners’ interest in net income		71,871		54,459		74,171	84,466
Less dilutive impact of theoretical distribution of earnings under FASB ASC 260-10-45-60		12,934		9,444		12,294	14,423
Limited Partner’s interest in net income under FASB ASC 260-10-45-60		$ 58,937		$ 45,015		$ 61,877	$ 70,043
Per unit data:
Basic and diluted net income available to limited partners		$ 1.40		$ 0.98		$ 1.42	$ 1.51
Less dilutive impact of theoretical distribution of earnings under FASB ASC 260-10-45-60		0.25		0.17		0.23	0.25
Limited Partner’s interest in net income under FASB ASC 260-10-45-60	[1]	$ 1.15		$ 0.81		$ 1.19	$ 1.26
Weighted average number of Limited Partner units outstanding		51,427		55,642		52,174	55,766

[1] See Note 15 - Earnings Per Limited Partner Unit.