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, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Basic and Diluted Earnings Per Limited Partner:
Net income		$ 39,704	$ 55,209	$ 57,979	$ 67,267
Less General Partner's interest in net income		233	313	338	381
Net income available to limited partners		39,471	54,896	57,641	66,886
Less dilutive impact of theoretical distribution of earnings under FASB ASC 260-10-45-60		6,607	9,617	9,115	10,960
Limited Partner's interest in net income under FASB ASC 260-10-45-60		$ 32,864	$ 45,279	$ 48,526	$ 55,926
Per unit data:
Basic and diluted net income available to limited partners		$ 0.71	$ 0.96	$ 1.03	$ 1.17
Less dilutive impact of theoretical distribution of earnings under FASB ASC 260-10-45-60		0.12	0.17	0.16	0.19
Limited Partner's interest in net income under FASB ASC 260-10-45-60	[1]	$ 0.59	$ 0.79	$ 0.87	$ 0.98
Weighted average number of Limited Partner units outstanding		55,888	57,242	55,888	57,262

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