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, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Basic and Diluted Earnings Per Limited Partner:
Net income		$ 54,778	$ 39,704	$ 84,960	$ 57,979
Less General Partner’s interest in net income		319	233	494	338
Limited Partners’ interest in net income		54,459	39,471	84,466	57,641
Less dilutive impact of theoretical distribution of earnings under FASB ASC 260-10-45-60		9,444	6,607	14,423	9,115
Limited Partner’s interest in net income under FASB ASC 260-10-45-60		$ 45,015	$ 32,864	$ 70,043	$ 48,526
Per unit data:
Basic and diluted net income available to limited partners		$ 0.98	$ 0.71	$ 1.51	$ 1.03
Less dilutive impact of theoretical distribution of earnings under FASB ASC 260-10-45-60		0.17	0.12	0.25	0.16
Limited Partner’s interest in net income under FASB ASC 260-10-45-60	[1]	$ 0.81	$ 0.59	$ 1.26	$ 0.87
Weighted average number of Limited Partner units outstanding		55,642	55,888	55,766	55,888

[1] See Note 13 - Earnings Per Limited Partner Unit.