Basis of Presentation - Change in Accounting Policy (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Jan. 01, 2016
Accounting Changes
Accumulated deficit	$ (228,697)	$ (212,870)
Adjustments for adoption of guidance | Accounting Standards Update 2014-09-09, Revenue from Contracts with Customers
Accounting Changes
Billing retention recognized at time of transfer of control (as a percent)	10.00%
Accumulated deficit		$ 506	$ 6,900