Revenue from Contract with Customers - Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Unbilled receivable	$ 21,975	$ 21,975	$ 20,482
Contract with Customer, Liability, Revenue Recognized	6,700	9,700
Revenue, Remaining Performance Obligation, Amount	3,200,000	3,200,000
ASUS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Unbilled receivable	10,399	10,399	10,467
Receivable from the U.S. government	64,399	64,399	64,819
Contract assets	20,458	20,458	15,631
Contract liabilities	$ 10,569	$ 10,569	$ 11,167
Maximum | ASUS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Explanation		P48Y
Minimum | ASUS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Explanation		P34Y
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