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Revenues - Contract assets (Details) - USD ($)
6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Revenue from Contract with Customer [Abstract]    
Contract with customer, liability $ 0 $ 0
Change in Contract with Customer, Asset [Roll Forward]    
Beginning balance 6,200,000 5,100,000
Contract asset additions 7,900,000 11,700,000
Reclassification to accounts receivable, billed to customers (7,600,000) (10,500,000)
Ending balance $ 6,500,000 $ 6,300,000