Quarterly Financial Results (unaudited) (Details) (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 Results (unaudited) [Abstract]
Revenues	$ 377,872	$ 345,321	$ 331,306	$ 319,448	$ 323,740	$ 281,856	$ 276,989	$ 275,953	$ 1,373,947	$ 1,158,538	$ 1,023,586
Cost of revenues	111,893	109,995	107,457	102,566	102,544	93,284	89,647	89,068	431,911	374,543	303,403
Net income	$ 68,292	$ 48,231	$ 44,239	$ 43,227	$ 60,081	$ 42,285	$ 47,921	$ 50,617	$ 203,989	$ 200,904	$ 171,220
Basic net income per share (in dollars per share)	$ 0.38	$ 0.27	$ 0.25	$ 0.24	$ 0.34	$ 0.23	$ 0.26	$ 0.27
Diluted net income per share (in dollars per share)	$ 0.38	$ 0.27	$ 0.24	$ 0.24	$ 0.33	$ 0.23	$ 0.25	$ 0.26
Basic (in shares)	177,479	177,455	178,547	178,120	178,916	183,085	186,612	186,849	177,900	183,866	177,309
Diluted (in shares)	181,768	181,053	181,817	182,342	182,956	185,704	190,179	191,383	181,749	187,556	190,650