Reconciliation of Numerators and Denominators of Basic and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Earnings Per Share Disclosure [Line Items]
Numerator for diluted and basic earnings per share-Net income:	$ 20,012	$ 7,745	$ 50,009	$ 23,754
Denominator-shares:
Basic weighted-average shares	35,331	40,010	35,685	39,966
Effect of dilutive securities	1,221	608	1,036	562
Diluted weighted-average shares	36,552	40,618	36,721	40,528
Earnings per share
Basic	$ 0.57	$ 0.19	$ 1.40	$ 0.59
Diluted	$ 0.55	$ 0.19	$ 1.36	$ 0.59