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Revenue from Contract with Customer - Assets and Liabilities (Details) - ASUS - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]    
Receivable from the U.S. government $ 48,383 $ 40,150
Contract assets 22,565 30,388
Contract liabilities $ 6,583 $ 3,911