Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share [Abstract]
Net income	$ 387,744	$ 196,218	$ 636,675	$ 377,603
Interest on Convertible Debt, Net of Tax	0	0	0	373
Net Income (Loss) Available to Common Stockholders, Diluted	$ 387,744	$ 196,218	$ 636,675	$ 377,976
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Weighted average shares outstanding - basic	106,034	104,633	105,804	104,462
Effect of dilutive securities (in shares):
Stock options	9,602	10,141	9,310	10,433
Restricted stock	501	473	493	471
Incremental Common Shares Attributable to Dilutive Effect of Conversion of Debt Securities	0	0	0	121
Warrants	0	984	0	1,130
Dilutive potential shares	10,103	11,598	9,803	12,155
Weighted average shares - diluted (in shares)	116,137	116,231	115,607	116,617
Earnings Per Share, Basic and Diluted [Abstract]
Net income per share - basic	$ 3.66	$ 1.88	$ 6.02	$ 3.61
Net income per share - diluted (in dollars per share)	$ 3.34	$ 1.69	$ 5.51	$ 3.24
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of shares	7,959	7,904	11,055	7,830
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of shares	0	20	16	19
Convertible Debt Securities [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of shares	0	96	0	0