Accounts Receivable - Accounts receivable, net (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable
Billed	$ 191,579	$ 266,660
Unbilled	52,052	56,408
Other receivables	4,124	5,278
Total	247,755	328,346
Less: Allowance for doubtful accounts	(5,181)	(4,813)
Accounts receivable, net	$ 242,574	$ 323,533