Repurchases of Common Stock (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Equity [Abstract]
Shares repurchased in the open market (in shares)	267	465
Shares acquired through employee surrender for statutory tax withholding (in shares)	36	48
Total shares repurchased (in shares)	303	513
Cost of shares repurchased in the open market	$ 53,862	$ 86,188
Cost of shares for employee surrenders	7,403	8,555
Total cost of shares	$ 61,265	$ 94,743
Average cost per share - open market repurchases (in dollars per share)	$ 201.41	$ 185.23
Average cost per share - employee surrenders (in dollars per share)	206.35	178.83
Average cost per share - total (in dollars per share)	$ 202.00	$ 184.63