Summary of Significant Accounting Policies - Research and Development (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Research and Development
Customer-funded service sales	$ 4,563	$ 2,621	$ 1,400
Customer-funded costs included in costs of service sales	$ 3,087	$ 1,510	$ 498