Revenue - Contract balances (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Revenue
Accounts receivable	$ 42,632	$ 43,041
Contract assets	17,009	18,189
Contract liabilities	72,897	76,285
Other Current Liabilities
Revenue
Contract liabilities	15,300	20,100
Services performed on fuel cell systems and related infrastructure
Revenue
Contract liabilities	$ 57,600	$ 56,200