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Accounts Receivable (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018
Accounts Receivable.
Billed receivables	$ 222,168				$ 226,252
Unbilled receivables	34,135				39,498
Other	10,072				9,421
Less: Allowance for doubtful accounts	(4,975)				(4,359)
Accounts Receivable, net	$ 261,400	$ 260,438	$ 266,660	$ 278,064	$ 270,812	$ 253,986	$ 262,260	$ 238,680