Revenue - Composition of Contract Assets and Contract Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Contract assets, current
Unbilled accounts receivable	$ 3,313	$ 2,813
Contract assets, long-term
Unbilled accounts receivable	10,148	8,440
Total contract assets	13,461	11,253	$ 8,439	$ 0
Contract liabilities, current
Deferred revenue, current	7,283	7,143
Customer advance payment	326	180
Total	7,609	7,323
Contract liabilities, long-term
Deferred revenue, long-term	12,706	14,732
Total contract liabilities	$ 20,315	$ 22,055	$ 21,676	$ 19,164