Stockholders' Equity - Common Stock Purchased Including Purchase of Common Stock for Tax Withholdings (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Total Number of Shares Purchased (shares)	0	0	572,550	0	16,719	13,884	65,791	291,861	189,438	502,942	177,330	27,467	572,550	388,255	897,177
Amount	$ 0	$ 0	$ 10,000	$ 0	$ 274	$ 228	$ 1,078	$ 4,893	$ 3,196	$ 8,309	$ 3,002	$ 465	$ 10,000	$ 6,473	$ 14,972