Goodwill and Intangible Assets - Finite-lived Intangible Assets Roll Forward (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Finite-lived Intangible Assets, Gross [Roll Forward]
Intangible assets, January 1	$ 35,240	$ 34,362
Additions	0	878
Total intangible assets, end of period	35,240	35,240
Accumulated amortization, January 1
Accumulated amortization, January 1	(23,955)	(19,220)
Current year amortization	(1,694)	(2,613)
Total accumulated amortization	(25,649)	(21,833)
Net intangible assets, end of period	$ 9,591	$ 13,407