Segment and Related Information - Revenue by Client Type (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Consulting Fee Revenue and Total Revenue By Client Type:
Total revenue	$ 109,571	$ 96,604	$ 317,537	$ 285,237
U.S. federal government
Consulting Fee Revenue and Total Revenue By Client Type:
Total revenue	4,870	3,506	12,576	11,636
U.S. state, regional and local governments
Consulting Fee Revenue and Total Revenue By Client Type:
Total revenue	35,922	30,648	103,331	90,934
Foreign governments
Consulting Fee Revenue and Total Revenue By Client Type:
Total revenue	23,710	23,401	72,000	75,137
Private sector
Consulting Fee Revenue and Total Revenue By Client Type:
Total revenue	$ 45,069	$ 39,049	$ 129,630	$ 107,530
Customer Concentration Risk | Revenue Benchmark
Consulting Fee Revenue and Total Revenue By Client Type:
Percentage of total revenue	100.00%	100.00%	100.00%	100.00%
Customer Concentration Risk | Revenue Benchmark | U.S. federal government
Consulting Fee Revenue and Total Revenue By Client Type:
Percentage of total revenue	4.40%	3.60%	4.00%	4.10%
Customer Concentration Risk | Revenue Benchmark | U.S. state, regional and local governments
Consulting Fee Revenue and Total Revenue By Client Type:
Percentage of total revenue	32.80%	31.70%	32.50%	31.90%
Customer Concentration Risk | Revenue Benchmark | Foreign governments
Consulting Fee Revenue and Total Revenue By Client Type:
Percentage of total revenue	21.60%	24.20%	22.70%	26.30%
Customer Concentration Risk | Revenue Benchmark | Private sector
Consulting Fee Revenue and Total Revenue By Client Type:
Percentage of total revenue	41.20%	40.50%	40.80%	37.70%