Contract Assets and Liabilities - Summary of contract liabilities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Contractors [Line Items]
Deferred revenue	$ 126,446		$ 102,673
Accrued loss provision	2,256		2,813
Contract liabilities	128,702		$ 105,486
Changes in contract liabilities	23,271	$ (5,035)
Net Period Change [Member]
Contractors [Line Items]
Changes in deferred revenue	23,773
Changes in accrued loss provision	(557)
Changes in contract liabilities	$ 23,216