Net Income Allocation and Per Unit Data (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended				6 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Mar. 31, 2015		Mar. 31, 2014
Basic and Diluted Earnings Per Limited Partner:
Net income	$ 75,687		$ 52,216		$ 91,241		$ 71,504
Less General Partner's interest in net income	428		294		516		403
Net income available to limited partners	75,259		51,922		90,725		71,101
Less dilutive impact of theoretical distribution of earnings under FASB ASC 260-10-45-60	13,616		9,065		15,705		11,922
Limited Partner's interest in net income under FASB ASC 260-10-45-60	$ 61,643		$ 42,857		$ 75,020		$ 59,179
Per unit data:
Basic and diluted net income available to limited partners	1.31		0.90		1.58		1.24
Less dilutive impact of theoretical distribution of earnings under FASB ASC 260-10-45-60	0.23		0.15		0.27		0.21
Limited Partner's interest in net income under FASB ASC 260-10-45-60	$ 1.08	[1]	$ 0.75	[1]	$ 1.31	[1]	$ 1.03	[1]
Weighted average number of Limited Partner units outstanding	57,282		57,468		57,288		57,490

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