Accounts Receivable and Allowance for Expected Credit Losses - Writeoff (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Allowance For Doubtful Accounts And Unbilled Services [Abstract]
Provision for expected credit losses	$ 19,692	$ 19,602	$ 17,872
Write-offs	$ 24,717	$ 12,734	$ 21,465