Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 477,980	$ 248,931
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Weighted average shares outstanding - basic	107,648	105,572
Effect of dilutive securities (in shares):
Stock options	7,244	9,050
Restricted stock	14	484
Dilutive potential shares	7,258	9,534
Weighted average shares - diluted (in shares)	114,906	115,106
Earnings Per Share, Basic and Diluted [Abstract]
Net income per share - basic	$ 4.44	$ 2.36
Net income per share - diluted (in dollars per share)	$ 4.16	$ 2.16
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of shares	14,878	11,535
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of shares	57	18