Net Income Per Share Schedule of Basic and Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income - basic and diluted	$ 551,412	$ 387,744	$ 1,029,392	$ 636,675
Weighted average shares outstanding - basic (in shares)	107,800	106,034	107,724	105,804
Effect of dilutive securities:
Stock options (in shares)	6,664	9,602	6,959	9,310
Restricted stock (in shares)	13	501	14	493
Dilutive potential shares (in shares)	6,677	10,103	6,973	9,803
Weighted average shares - diluted (in shares)	114,477	116,137	114,697	115,607
Net income per share - basic (in dollars per share)	$ 5.12	$ 3.66	$ 9.56	$ 6.02
Net income per share - diluted (in dollars per share)	$ 4.82	$ 3.34	$ 8.97	$ 5.51