Summary of Significant Accounting Policies - Research and Development (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Research and Development
Customer-funded service sales	$ 2,621	$ 1,400	$ 3,002
Customer-funded costs included in costs of service sales	$ 1,510	$ 498	$ 1,546