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Revenue - Change in Contract Balances (Details)
$ in Thousands
 6 Months Ended
Jun. 30, 2021
USD ($)
Contract Assets  
Contract with customer, asset, net, beginning balance $ 26,780
Payments received for services billed in prior period (26,780)
Services provided and billed in current period 118,835
Payments received for services billed in current period (99,802)
Contract with customer, asset, net, ending balance 19,033
Contract Liabilities  
Contract with customer, liability, beginning balance (22,856)
Amortization of deferred commissioning period revenue 2,043
Contract with customer, liability, ending balance $ (20,813)