Common Stock (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Common stock repurchases, value	$ 40,791	$ 20,520	$ 52,103
Common Stock [Member]
Class of Stock [Line Items]
Treasury shares issued (in shares)	0	0	0
Employee stock plans (in shares)	341,837	621,259	841,164
Treasury Stock [Member]
Class of Stock [Line Items]
Common stock repurchases (in shares)	677,100	550,994	1,352,596
Common stock repurchases, value	$ 40,791	$ 20,520	$ 52,103