Repurchases of Common Stock (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Equity [Abstract]
Shares repurchased in the open market (in shares)	330	302	683	1,284
Shares acquired through employee surrender for statutory tax withholding (in shares)	1	1	38	50
Total shares repurchased (in shares)	331	303	721	1,334
Cost of shares repurchased in the open market	$ 91,051	$ 73,247	$ 165,198	$ 265,209
Cost of shares for employee surrenders	277	390	7,849	9,110
Total cost of shares	$ 91,328	$ 73,637	$ 173,047	$ 274,319
Average cost per share - open market repurchases (in dollars per share)	$ 275.50	$ 242.78	$ 241.56	$ 206.59
Average cost per share - employee surrenders (in dollars per share)	286.65	253.34	209.20	181.68
Average cost per share - total (in dollars per share)	$ 275.53	$ 242.83	$ 239.87	$ 205.65