Quarterly Operating Results (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Quarterly Financial Information [Line Items]
Revenues	$ 158,123	$ 167,083	$ 149,669	$ 150,830	$ 119,584	$ 118,517	$ 111,532	$ 123,449	$ 625,705	$ 473,082	$ 341,872
Gross profit	64,848	72,693	62,118	62,056	46,366	44,499	41,241	42,565	261,715	174,671	120,503
Net income	$ 33,117	$ 36,911	$ 29,944	$ 30,164	$ 20,815	$ 20,175	$ 18,734	$ 18,992	$ 130,136	$ 78,716	$ 52,810
Earnings per share:
Basic	$ 1.43	$ 1.59	$ 1.29	$ 1.30	$ 0.90	$ 0.87	$ 0.81	$ 0.82	$ 5.62	$ 3.41	$ 2.27
Diluted	$ 1.43	$ 1.59	$ 1.29	$ 1.30	$ 0.90	$ 0.87	$ 0.81	$ 0.82	$ 5.62	$ 3.40	$ 2.27