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Revenue from Contract with Customers - Assets and Liabilities (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2021
Dec. 31, 2020
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]      
Unbilled receivable $ 25,517 $ 25,517 $ 25,836
Contract with Customer, Liability, Revenue Recognized 0 838  
Revenue, Remaining Performance Obligation, Amount 3,300,000 3,300,000  
ASUS      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]      
Unbilled receivable 11,041 11,041 14,924
Receivable from the U.S. government 81,854 81,854 74,670
Contract assets 10,748 10,748 10,257
Contract liabilities $ 555 $ 555 $ 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   33  
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