Stock-Based Compensation - Issued Shares of Common Stock (Details) - 401(k) Plan [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock issued (shares)	102,874	172,650	179,865
Match contribution value	$ 912	$ 953	$ 1,124