Revenue from Contract with Customers - Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Unbilled receivable	$ 25,086	$ 25,086	$ 20,482
Contract with Customer, Liability, Revenue Recognized	7,100	10,900
Revenue, Remaining Performance Obligation, Amount	3,200,000	3,200,000
ASUS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Unbilled receivable	12,955	12,955	10,467
Receivable from the U.S. government	70,106	70,106	64,819
Contract assets	10,086	10,086	15,631
Contract liabilities	$ 8,670	$ 8,670	$ 11,167
Maximum | ASUS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Explanation		48
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