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, 2014		Mar. 31, 2013		Mar. 31, 2014		Mar. 31, 2013
Basic and Diluted Earnings Per Limited Partner:
Net income	$ 52,216		$ 41,679		$ 71,504		$ 51,431
Less General Partners' interest in net income	294		225		403		278
Net income available to limited partners	51,922		41,454		71,101		51,153
Less dilutive impact of theoretical distribution of earnings under FASB ASC 260-10-45-60	9,065		6,993		11,922		7,991
Limited Partner's interest in net income under FASB ASC 260-10-45-60	$ 42,857		$ 34,461		$ 59,179		$ 43,162
Per unit data:
Basic and diluted net income available to limited partners	0.90		0.69		1.24		0.85
Less dilutive impact of theoretical distribution of earnings under FASB ASC 260-10-45-60	0.15		0.11		0.21		0.13
Limited Partner's interest in net income under FASB ASC 260-10-45-60	$ 0.75	[1]	$ 0.58	[1]	$ 1.03	[1]	$ 0.72	[1]
Weighted average number of Limited Partner units outstanding	57,468		59,837		57,490		60,192

[1] See Note 14 Earnings Per Limited Partner Unit.