Net Income per Share (Schedule of Components Used in Diluted and Basic Income Per Common Share) (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
Numerator [Abstract]
Net income	$ 60,081	$ 42,285	$ 47,921	$ 50,617	$ 52,510	$ 39,709	$ 38,123	$ 40,878	$ 200,904	$ 171,220	$ 145,913
Add back of interest expense on 1% convertible senior notes (net of tax)									0	1,059	1,746
Numerator for diluted net income per common share									$ 200,904	$ 172,279	$ 147,659
Denominator [Abstract]
Denominator for basic net income per common share	178,916	183,085	186,612	186,849	183,362	181,457	173,317	171,101	183,866	177,309	171,425
Effect of dilutive securities:
Stock options									2,550	3,821	2,805
Effect of escrow contingencies									0	254	342
Net cash used in financing activities									1,140	1,395	1,153
Assumed conversion of 1% convertible senior notes									0	7,871	12,933
Denominator for diluted net income per common share	182,956	185,704	190,179	191,383	191,837	191,271	190,479	189,013	187,556	190,650	188,658
Basic net income per common share									$ 1.09	$ 0.97	$ 0.85
Diluted net income per common share									$ 1.07	$ 0.90	$ 0.78