Income Taxes - Deferred Tax Assets (Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred Tax Assets
Allowance for bad debt	$ 1,687	$ 888
Accrued paid time off	2,423	1,671
Foreign net operating loss (NOL) carry forward	957	866
State net operating loss (NOL) carry forward	714	714
Stock option compensation	2,308	2,666
Deferred rent	3,091	3,129
Deferred compensation	4,598	3,902
Foreign tax credits	5,882	4,508
Foreign exchange	5,370	3,579
Foreign deferred	650	200
Accrued bonus	4,126	884
Accrued liabilities and other	5,701	4,434
Deferred Tax Assets, gross, before valuation allowance	39,615	29,467
Less: Valuation Allowance	(6,839)	(5,374)	$ (5,112)	$ (1,636)
Total Deferred Tax Assets	32,776	24,093
Deferred Tax Liabilities
Retention	(1,003)	(1,395)
Prepaid expenses	(1,089)	(1,451)
Payroll taxes	(489)	(593)
Unbilled revenue	(1,451)	(2,691)
Depreciation	(1,731)	(3,112)
Amortization	(60,392)	(52,076)
Deferred gain and other	(951)	(396)
Total Deferred Tax Liabilities	(67,106)	(61,714)
Total Net Deferred Tax Liability	(34,330)	(37,621)
State and Local Jurisdiction
Deferred Tax Assets
Tax credits	$ 2,108	$ 2,026