Contract Assets and Contract Liabilities - Current and Long-term Contract Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract liabilities, current	$ 4,843	$ 2,252
Contract liabilities, non-current	2,378	$ 4,442
within 12 to 24 months
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract liabilities, non-current	1,994
greater than 24 months
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract liabilities, non-current	$ 384