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, 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
Numerator [Abstract]
Net income	$ 68,292	$ 48,231	$ 44,239	$ 43,227	$ 60,081	$ 42,285	$ 47,921	$ 50,617	$ 203,989	$ 200,904	$ 171,220
Add back of interest expense on 1% convertible senior notes (net of tax)									0	0	1,059
Numerator for diluted net income per common share									$ 203,989	$ 200,904	$ 172,279
Denominator [Abstract]
Denominator for basic net income per common share (in shares)	177,479	177,455	178,547	178,120	178,916	183,085	186,612	186,849	177,900	183,866	177,309
Effect of dilutive securities:
Stock options									2,182	2,550	3,821
Effect of escrow contingencies									0	0	254
Net cash used in financing activities									1,667	1,140	1,395
Assumed conversion of 1% convertible senior notes									0	0	7,871
Denominator for diluted net income per common share (in shares)	181,768	181,053	181,817	182,342	182,956	185,704	190,179	191,383	181,749	187,556	190,650
Basic net income per common share (in dollars per share)									$ 1.15	$ 1.09	$ 0.97
Diluted net income per common share (in dollars per share)									$ 1.12	$ 1.07	$ 0.90