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Revenue from Contract with Customers - Assets and Liabilities (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2021
Dec. 31, 2020
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]      
Unbilled receivable $ 29,569 $ 29,569 $ 25,836
Contract with Customer, Liability, Revenue Recognized 186 838  
Revenue, Remaining Performance Obligation, Amount 3,200,000 3,200,000  
ASUS      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]      
Unbilled receivable 16,347 16,347 14,924
Receivable from the U.S. government 78,655 78,655 74,670
Contract assets 11,172 11,172 10,257
Contract liabilities $ 688 $ 688 $ 1,800
Maximum | ASUS      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]      
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Explanation   47  
Minimum | ASUS      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]      
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Explanation   34  
Contracted Services [Member] | ASUS      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]      
Period of Fixed Price Contracts to Operate and Maintain Water Systems at Various Military Bases   50 years