Revenue - Contract balances (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Revenue
Accounts receivable	$ 91,359	$ 43,041
Contract assets	11,549	18,189
Contract liabilities	93,665	76,285
Other Current Liabilities
Revenue
Contract liabilities	21,400	20,100
Sales of fuel cell systems
Revenue
Contract liabilities	$ 72,300	$ 56,200