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Summary of Significant Accounting Policies - Significant Customers (Details)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
California      
Segment Reporting Information [Line Items]      
Percentage of revenues under contracts or subcontracts 13.00% 18.00% 21.00%
Texas      
Segment Reporting Information [Line Items]      
Percentage of revenues under contracts or subcontracts 10.00% 12.00% 13.00%