Income Taxes - Deferred Tax Assets (Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Assets
Allowance for bad debt	$ 1,321	$ 1,003
Accrued paid time off	1,437	1,624
Foreign net operating loss (NOL) carry forward	1,144	1,301
State net operating loss (NOL) carry forward	507	507
Stock option compensation	2,332	2,726
Deferred rent	3,127	3,355
Deferred compensation	3,348	3,238
Foreign tax credits	3,968	505
Foreign exchange	2,430	2,051
Foreign deferred	342
Accrued liabilities and other	4,079	3,272
Deferred Tax Assets, gross, before valuation allowance	26,076	21,367
Less: Valuation Allowance	(5,112)	(1,636)	$ (1,131)	$ (933)
Total Deferred Tax Assets	20,964	19,731
Deferred Tax Liabilities
Retention	(1,239)	(1,375)
Prepaid expenses	(1,301)	(1,045)
Payroll taxes	(495)	(489)
Unbilled revenue	(4,135)	(5,407)
Depreciation	(7,306)	(4,773)
Amortization	(46,051)	(39,993)
Deferred gain and other	(602)
Total Deferred Tax Liabilities	(61,129)	(53,082)
Total Net Deferred Tax Liability	(40,165)	(33,351)
State and Local Jurisdiction
Deferred Tax Assets
Tax credits	$ 2,041	$ 1,785