Stockholders' Equity (Details) - USD ($)		1 Months Ended	3 Months Ended	12 Months Ended
	Jun. 01, 2017	Aug. 31, 2015	Jun. 30, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	May 29, 2015	Jun. 05, 2012
Class of Stock [Line Items]
Preferred stock, authorized (in shares)				5,000,000	5,000,000
Preferred stock, issued (in shares)				0	0
Employee stock purchase (in shares)				1,949,117	1,312,812	2,989,324
Employee stock purchases				$ 48,621,000	$ 33,776,000	$ 65,238,000
Issuance of restricted stock awards (in shares)				166,103	132,344	166,042
Common stock, authorized (in shares)				187,500,000	187,500,000	125,000,000	187,500,000
Issuance of common stock						$ 29,964,000
Stock repurchase program, authorized amount								$ 50,000,000
Repurchase of common stock (in shares)			2,192,982
Repurchase of common stock			$ 50,000,000
Common stock held in treasury (in shares)				3,013,143	2,192,982
Common Stock [Member]
Class of Stock [Line Items]
Employee stock purchase (in shares)				1,949,000	1,313,000	2,989,000
Employee stock purchases				$ 2,000	$ 1,000	$ 3,000
Issuance of restricted stock awards (in shares)				166,000	132,000	166,000
Issuance of common stock (in shares)		944,537				945,000
Issuance of common stock		$ 30,000,000				$ 1,000
Common stock received upon settlement (in shares)				(820,000)
Interest Rate Contract [Member]
Class of Stock [Line Items]
Common stock received upon settlement (in shares)	820,161
Common stock received upon settlement (usd per share)	$ 48.79