Net Income (Loss) Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 173,519	$ 388,317	$ 387,744	$ 248,931	$ 253,115	$ 264,804	$ 196,218	$ 181,385	$ 1,198,511	$ 895,522	$ 636,056
Dilutive Securities, Effect on Basic Earnings Per Share, Dilutive Convertible Securities									0	397	0
Net Income (Loss) Attributable to Parent, Diluted									$ 1,198,511	$ 895,919	$ 636,056
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Weighted average shares outstanding - basic									106,338	104,719	103,061
Effect of dilutive securities (in shares):
Stock options									9,132	10,177	9,446
Restricted stock									484	474	477
Incremental Common Shares Attributable to Dilutive Effect of Conversion of Debt Securities									0	61	0
Warrants									0	936	2,246
Dilutive potential shares									9,616	11,648	12,169
Weighted average shares - diluted (in shares)									115,954	116,367	115,230
Earnings Per Share, Basic and Diluted [Abstract]
Net income per share - basic (in USD per shares)	$ 1.62	$ 3.64	$ 3.66	$ 2.36	$ 2.41	$ 2.53	$ 1.88	$ 1.74	$ 11.27	$ 8.55	$ 6.17
Net income per share - diluted (in USD per shares)	$ 1.50	$ 3.32	$ 3.34	$ 2.16	$ 2.19	$ 2.27	$ 1.69	$ 1.59	$ 10.34	$ 7.70	$ 5.52
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of shares									9,161	8,041	1,343
Restricted stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of shares									0	19	0
Convertible senior notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of shares									0	0	994