Net Income Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share [Abstract]
Net income	$ 92,735	$ 87,376	$ 158,178	$ 186,250
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Weighted average shares outstanding - basic	100,391	97,700	100,085	97,289
Effect of dilutive securities (in shares):
Stock options	9,359	10,291	9,615	10,296
Restricted stock	405	424	403	383
Warrants	2,877	2,645	3,018	2,337
Dilutive potential shares	12,641	13,360	13,036	13,016
Weighted average shares - diluted (in shares)	113,032	111,060	113,121	110,305
Earnings Per Share, Basic and Diluted [Abstract]
Net income per share - basic	$ 0.92	$ 0.89	$ 1.58	$ 1.91
Net income per share - diluted (in dollars per share)	$ 0.82	$ 0.79	$ 1.40	$ 1.69
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of shares	3,765	1,247	3,714	3,599
Convertible senior notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of shares	4,662	4,761	4,711	4,761