Calculations of Basic and Diluted Net Income per Share and Potential Common Shares (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Earnings Per Share Disclosure [Line Items]
Net income	$ 14,553	$ 11,138	$ 9,910	$ 5,008	$ 12,947	$ 10,031	$ 7,051	$ 2,399	$ 40,609	$ 32,428	$ 37,265
Weighted average common shares outstanding									41,665	42,035	42,059
Basic net income per share									$ 0.97	$ 0.77	$ 0.89
Weighted average stock options									122	154	320
Weighted average restricted stock units									535	561	594
Diluted weighted average shares									42,322	42,750	42,973
Diluted net income per share	$ 0.34	$ 0.26	$ 0.23	$ 0.12	$ 0.30	$ 0.24	$ 0.17	$ 0.06	$ 0.96	$ 0.76	$ 0.87