Repurchases of Common Stock (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Equity [Abstract]
Shares repurchased in the open market (in shares)	86	517	353	982
Shares acquired through employee surrender for statutory tax withholding (in shares)	1	1	37	49
Total shares repurchased (in shares)	87	518	390	1,031
Cost of shares repurchased in the open market	$ 20,285	$ 105,774	$ 74,147	$ 191,962
Cost of shares for employee surrenders	169	165	7,572	8,720
Total cost of shares	$ 20,454	$ 105,939	$ 81,719	$ 200,682
Average cost per share - open market repurchases (in dollars per share)	$ 235.94	$ 204.69	$ 209.81	$ 195.47
Average cost per share - employee surrenders (in dollars per share)	249.77	215.36	207.16	179.41
Average cost per share - total (in dollars per share)	$ 236.04	$ 204.71	$ 209.56	$ 194.71