Stockholders' Equity - Stock Repurchases (Details) - USD ($)		3 Months Ended								12 Months Ended
	Mar. 29, 2016	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, 2017	Dec. 31, 2016
Stock Repurchases
Number of shares repurchased (shares)		0	0	0	983,864	2,062,124	621,528	177,980	0	983,864	2,861,632
Repurchase amount		$ 0	$ 0	$ 0	$ 2,406,000	$ 4,515,000	$ 1,183,000	$ 339,000	$ 0	$ 2,406,000	$ 6,037,000
Common Stock
Stock Repurchases
Stock repurchase program, period	18 months
Remaining repurchase amount authorized		$ 6,557,000								$ 6,557,000
Common Stock | Maximum
Stock Repurchases
Amount authorized	$ 15,000,000