Concentration of Risk and Enterprise-Wide Disclosures Narrative (Details) - Customer concentration risk - Contract revenues	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2017
Concentration Risk [Line Items]
Concentration risk, percentage	100.00%	100.00%		100.00%	100.00%
Local Governments
Concentration Risk [Line Items]
Concentration risk, percentage	31.00%	13.00%		31.00%	15.00%
Local Governments | Customer One
Concentration Risk [Line Items]
Concentration risk, percentage	14.00%		11.00%
Private Companies
Concentration Risk [Line Items]
Concentration risk, percentage	55.00%	71.00%		57.00%	69.00%
Private Companies | Customer One
Concentration Risk [Line Items]
Concentration risk, percentage		16.00%		12.00%