Contract Assets and Liabilities - Summary of contract liabilities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Contractors [Line Items]
Deferred revenue	$ 134,194		$ 155,570
Accrued loss provision	2,793		2,826
Contract liabilities	136,987		$ 158,396
Contract liabilities, Change	(21,433)	$ 23,271
Change
Contractors [Line Items]
Deferred revenue, Change	(21,376)
Accrued loss provision, Change	(33)
Contract liabilities, Change	$ (21,409)