UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Revenue	$ 358,624	$ 340,309	$ 704,917	$ 674,301
Direct costs	251,141	236,390	497,246	473,247
Gross profit	107,483	103,919	207,671	201,054
Selling, general and administrative expenses	82,448	82,506	167,040	167,184
Depreciation and amortization	1,937	2,053	3,945	4,103
Income from operations	23,098	19,360	36,686	29,767
Other expense, net	1,256	1,357	2,595	2,542
Income before income taxes	21,842	18,003	34,091	27,225
Income tax expense	5,570	6,859	8,644	10,179
Net income	16,272	11,144	25,447	17,046
Other comprehensive income (loss):
Change in fair value of interest rate swap, net of tax	180	(112)	697	(112)
Defined benefit pension plans, net of tax	0	(4)	0	(9)
Comprehensive income	$ 16,452	$ 11,028	$ 26,144	$ 16,925
Earnings per share – basic (in dollars per share)	$ 0.66	$ 0.44	$ 1.03	$ 0.68
Earnings per share – diluted (in dollars per share)	$ 0.65	$ 0.44	$ 1.01	$ 0.67
Weighted average shares outstanding – basic (in shares)	24,705	25,273	24,744	25,248
Weighted average shares outstanding – diluted (in shares)	25,178	25,482	25,142	25,542
Dividends declared per share (in dollars per share)	$ 0.12	$ 0.12	$ 0.24	$ 0.24