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Revenue from Contract with Customers - Assets and Liabilities (Details) - ASUS - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]    
Receivable from the U.S. government $ 60,011 $ 61,126
Contract assets 29,460 24,447
Contract liabilities $ 10,426 $ 7,530