Treasury Stock (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Shares available for repurchase (in shares)	0
Shares repurchased (in shares)	680,000	198,000	1,817,000
Cost of shares repurchased	$ 39,085	$ 12,647	$ 89,395