Intangible Assets and Goodwill - Amortization expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Intangible Assets and Goodwill
Amortization expense for intangible assets	$ 896	$ 997	$ 1,013
Estimated future amortization expense of intangible assets
2018	900
2019	900
2020	900
2021	900
2022	$ 900