Summary of Significant Accounting Policies - Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 33,872	$ 31,343	$ 36,747
Depreciation	6,725	7,608	7,193
Amortization of Intangible Assets	$ 4,312	$ 4,956	$ 5,526