Quarterly Financial Results (unaudited) (Details) (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 Results (unaudited) [Abstract]
Revenues	$ 323,740	$ 281,856	$ 276,989	$ 275,953	$ 284,688	$ 253,551	$ 245,318	$ 240,029	$ 1,158,538	$ 1,023,586	$ 859,773
Cost of revenues	102,544	93,284	89,647	89,068	86,277	77,812	71,840	67,474	374,543	303,403	249,938
Net income	$ 60,081	$ 42,285	$ 47,921	$ 50,617	$ 52,510	$ 39,709	$ 38,123	$ 40,878	$ 200,904	$ 171,220	$ 145,913
Basic net income per share	$ 0.34	$ 0.23	$ 0.26	$ 0.27	$ 0.29	$ 0.22	$ 0.22	$ 0.24
Diluted net income per share	$ 0.33	$ 0.23	$ 0.25	$ 0.26	$ 0.27	$ 0.21	$ 0.20	$ 0.22
Basic	178,916	183,085	186,612	186,849	183,362	181,457	173,317	171,101	183,866	177,309	171,425
Diluted	182,956	185,704	190,179	191,383	191,837	191,271	190,479	189,013	187,556	190,650	188,658