Selected Quarterly Financial Data (Schedule of Selected Quarterly Financial Data) (Detail) (USD $) In Thousands, except 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 Data [Line Items]
Revenues, net	$ 202,617	$ 186,932	$ 171,820	$ 146,165	$ 140,160	$ 134,213	$ 134,213	$ 111,005	$ 707,534	$ 519,591	$ 433,841
Operating income	93,171	85,834	81,448	64,475	54,232	61,723	59,892	50,487	324,928	226,334	170,493
Net income	$ 60,071	$ 59,648	$ 54,401	$ 42,079	$ 37,771	$ 40,514	$ 36,715	$ 32,335	$ 216,199	$ 147,335	$ 107,896
Basic earnings per share	$ 0.72	$ 0.71	$ 0.65	$ 0.51	$ 0.46	$ 0.50	$ 0.46	$ 0.40	$ 2.59	$ 1.83	$ 3.00
Diluted earnings per share	$ 0.70	$ 0.69	$ 0.63	$ 0.49	$ 0.45	$ 0.48	$ 0.44	$ 0.39	$ 2.52	$ 1.76	$ 1.34
Basic weighted average shares outstanding	83,378	84,002	83,294	82,565	81,512	80,819	80,151	79,937	83,328	80,610	35,434
Diluted weighted average shares outstanding	85,750	86,224	85,737	85,164	84,035	83,649	83,548	83,378	85,736	83,654	80,751