Fair Value of Financial Instruments and Concentrations of Credit Risk - Accounts Receivable (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Trade receivables:
Gross trade receivables	$ 133,717	$ 207,554
Allowance for credit losses	(5,024)	(6,007)	$ (10,034)	$ (9,457)
Net trade receivables	128,693	201,547
Income tax receivables	6,545	7,393
Other receivables	5,807	7,774
Total receivables, net	$ 141,045	$ 216,714