Earnings Per Share - Schedule of Computation of Basic and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net income	$ 25,841	$ 38,169	$ 33,331	$ 21,857	$ 34,125	$ 40,501	$ 34,670	$ 13,874	$ 119,198	$ 123,170	$ 111,830
Denominator for basic earnings per share - weighted average shares									38,705	39,650	40,337
Effect of dilutive securities: Employee stock options and restricted units									425	586	654
Denominator for diluted earnings per share - Adjusted weighted average shares and the effect of dilutive securities									39,130	40,236	40,991
Net income per share - basic	$ 0.67	$ 0.99	$ 0.86	$ 0.56	$ 0.86	$ 1.03	$ 0.87	$ 0.35	$ 3.08	$ 3.11	$ 2.77
Net income per share - diluted	$ 0.67	$ 0.98	$ 0.85	$ 0.55	$ 0.85	$ 1.01	$ 0.86	$ 0.34	$ 3.05	$ 3.06	$ 2.73