Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenues before reimbursements	$ 939,639	$ 938,746	$ 833,808
Reimbursements	92,688	95,734	85,678
Total revenues	1,032,327	1,034,480	919,486
Cost of services before reimbursable expenses	654,586	631,935	571,894
Reimbursable expenses	92,688	95,734	85,678
Total cost of services	747,274	727,669	657,572
General and administrative expenses	166,922	168,954	147,462
Depreciation expense	28,826	27,742	23,612
Amortization expense	8,960	11,507	8,613
Other operating costs (benefit):
Contingent acquisition liability adjustments, net	2,213	1,330	(13,047)
Office consolidation, net	242	542	2,766
Loss on disposition of assets			283
Deferred debt issuance costs write off	145
Other impairment			98
Other costs	1,620
Operating income	76,125	96,736	92,127
Interest expense	4,970	5,235	4,916
Interest income	(345)	(141)	(250)
Other (income) expense, net	919	(1,769)	(692)
Income before income tax expense	70,581	93,411	88,153
Income tax (benefit) expense	(4,371)	35,313	27,808
Net income	$ 74,952	$ 58,098	$ 60,345
Basic net income per share	$ 1.61	$ 1.23	$ 1.26
Shares used in computing basic per share data	46,593	47,343	47,906
Diluted net income per share	$ 1.55	$ 1.19	$ 1.23
Shares used in computing diluted per share data	48,226	48,813	49,224
Net income	$ 74,952	$ 58,098	$ 60,345
Other comprehensive income (loss), net of tax
Unrealized net gain (loss), foreign currency translation	5,858	(8,720)	(4,473)
Unrealized net gain (loss) on interest rate derivatives	100	(72)	(317)
Reclassification adjustment on interest rate derivatives included in interest expense and income tax expense	70	146	314
Other comprehensive gain (loss), net of tax	6,028	(8,646)	(4,476)
Total comprehensive income, net of tax	$ 80,980	$ 49,452	$ 55,869