Quarterly Operating Results (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information [Line Items]
Revenues	$ 153,622	$ 151,134	$ 177,614	$ 163,166	$ 158,123	$ 167,083	$ 149,669	$ 150,830	$ 645,536	$ 625,705	$ 473,082
Gross profit	45,638	50,150	64,253	63,464	64,848	72,693	62,118	62,056	223,505	261,715	174,671
Net income	$ 19,827	$ 23,898	$ 31,917	$ 30,291	$ 33,117	$ 36,911	$ 29,944	$ 30,164	$ 105,933	$ 130,136	$ 78,716
Earnings per share:
Basic	$ 0.86	$ 1.04	$ 1.38	$ 1.31	$ 1.43	$ 1.59	$ 1.29	$ 1.30	$ 4.59	$ 5.62	$ 3.41
Diluted	$ 0.86	$ 1.04	$ 1.38	$ 1.31	$ 1.43	$ 1.59	$ 1.29	$ 1.30	$ 4.59	$ 5.62	$ 3.40