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, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Basic and Diluted Earnings Per Limited Partner:
Net income		$ 55,209	$ 75,687	$ 67,267	$ 91,241
Less General Partner's interest in net income		313	428	381	516
Net income available to limited partners		54,896	75,259	66,886	90,725
Less dilutive impact of theoretical distribution of earnings under FASB ASC 260-10-45-60		9,617	13,616	10,960	15,705
Limited Partner's interest in net income under FASB ASC 260-10-45-60		$ 45,279	$ 61,643	$ 55,926	$ 75,020
Per unit data:
Basic and diluted net income available to limited partners		$ 0.96	$ 1.31	$ 1.17	$ 1.58
Less dilutive impact of theoretical distribution of earnings under FASB ASC 260-10-45-60		0.17	0.23	0.19	0.27
Limited Partner's interest in net income under FASB ASC 260-10-45-60	[1]	$ 0.79	$ 1.08	$ 0.98	$ 1.31
Weighted average number of Limited Partner units outstanding		57,242	57,282	57,262	57,288

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