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																12 Months Ended
	Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2010		Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
Earnings Per Share Disclosure [Line Items]
Numerator for basic and diluted earnings per share - net income	$ 21,246	[1]	$ 20,749	[1]	$ 20,012	[1]	$ 29,997	[1]	$ 24,619	[1]	$ 23,189	[1]	$ 7,745	[1]	$ 16,009	[1]	$ 92,004	$ 71,562	$ 64,457
Denominator - share:
Basic weighted-average shares	34,262		34,004		35,331		36,034		38,088		39,451		40,010		39,923		34,909	39,359	44,903
Effect of dilutive securities																	1,154	629	405
Diluted weighted-average shares	35,513		35,293		36,552		36,887		38,687		40,241		40,618		40,439		36,063	39,988	45,308
Earnings per share:
Basic	$ 0.62	[2]	$ 0.61	[2]	$ 0.57	[2]	$ 0.83	[2]	$ 0.65	[2]	$ 0.59	[2]	$ 0.19	[2]	$ 0.40	[2]	$ 2.64	$ 1.82	$ 1.44
Diluted	$ 0.60	[2]	$ 0.59	[2]	$ 0.55	[2]	$ 0.81	[2]	$ 0.64	[2]	$ 0.58	[2]	$ 0.19	[2]	$ 0.40	[2]	$ 2.55	$ 1.79	$ 1.42

[1] Net income includes restructuring and acquisition-related expenses of $5.1 million, $11.5 million, $0.9 million and $1.6 million for the quarters ended September 30, 2012, March 31, 2011 and December 31, 2010, respectively.

[2] Earnings per share is computed independently for each of the quarters presented. Therefore, the sum of the quarterly per share amounts may not equal the totals for the respective years.