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Construction Contract and Other Service Revenues (Narrative) (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Construction contract revenue | Revenue from Contract with Customer Benchmark | Concentration risk | United States Government
Concentration Risk [Line Items]
Percentage of revenue	74.00%	95.00%	98.00%

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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