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Revenue from Contract with Customers - Assets and Liabilities (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]    
Unbilled receivable $ 23,315 $ 25,836
Contract with Customer, Liability, Revenue Recognized 838  
Revenue, Remaining Performance Obligation, Amount 3,200,000  
ASUS    
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]    
Unbilled receivable 13,302 14,924
Receivable from the U.S. government 75,979 74,670
Contract assets 13,726 10,257
Contract liabilities $ 1,049 $ 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