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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Numerator [Abstract]
Net income (in dollars)	$ 61,895	$ 44,239	$ 133,382	$ 87,466
Denominator [Abstract]
Denominator for basic net income per common share	177,891	178,547	177,895	178,333
Effect of dilutive securities:
Denominator for diluted net income per common share	181,388	181,817	181,475	182,080
Basic net income per common share (in dollars per share)	$ 0.35	$ 0.25	$ 0.75	$ 0.49
Diluted net income per common share (in dollars per share)	$ 0.34	$ 0.24	$ 0.73	$ 0.48
Stock Option [Member]
Effect of dilutive securities:
Dilutive securities	1,724	2,092	1,750	2,259
Restricted Stock Units and Deferred Stock Units [Member]
Effect of dilutive securities:
Dilutive securities	1,773	1,178	1,830	1,488