Segment and Related Information - Revenue by Client Type (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Consulting Fee Revenue and Total Revenue By Client Type:
Total revenue	$ 94,449	$ 98,257	$ 187,757	$ 196,940
Percent of Total Revenue	100.00%	100.00%	100.00%	100.00%
U.S. federal government
Consulting Fee Revenue and Total Revenue By Client Type:
Total revenue	$ 3,922	$ 4,395	$ 8,532	$ 9,087
U.S. state, regional and local governments
Consulting Fee Revenue and Total Revenue By Client Type:
Total revenue	30,349	32,076	60,900	62,971
Foreign governments
Consulting Fee Revenue and Total Revenue By Client Type:
Total revenue	25,605	24,204	50,969	49,883
Private sector
Consulting Fee Revenue and Total Revenue By Client Type:
Total revenue	$ 34,573	$ 37,582	$ 67,356	$ 74,999
Customer Concentration Risk
Consulting Fee Revenue and Total Revenue By Client Type:
Percent of Total Revenue	100.00%	100.00%	100.00%	100.00%
Customer Concentration Risk | U.S. federal government
Consulting Fee Revenue and Total Revenue By Client Type:
Percent of Total Revenue	4.20%	4.50%	4.60%	4.60%
Customer Concentration Risk | U.S. state, regional and local governments
Consulting Fee Revenue and Total Revenue By Client Type:
Percent of Total Revenue	32.10%	32.60%	32.40%	32.00%
Customer Concentration Risk | Foreign governments
Consulting Fee Revenue and Total Revenue By Client Type:
Percent of Total Revenue	27.10%	24.60%	27.10%	25.30%
Customer Concentration Risk | Private sector
Consulting Fee Revenue and Total Revenue By Client Type:
Percent of Total Revenue	36.60%	38.30%	35.90%	38.10%