Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Revenue		$ 1,229,162	$ 1,185,097	$ 1,132,232
Direct costs		771,725	745,137	694,436
Operating costs and expenses
Indirect and selling expenses		346,440	328,048	329,159
Depreciation and amortization		17,691	16,638	16,222
Amortization of intangible assets		10,888	12,481	17,184
Total operating costs and expenses		375,019	357,167	362,565
Operating income		82,418	82,793	75,231
Interest expense		(8,553)	(9,470)	(10,072)
Other income (expense)		121	1,184	(1,559)
Income before income taxes		73,986	74,507	63,600
Provision for income taxes		11,110	27,923	24,231
Net income		$ 62,876	$ 46,584	$ 39,369
Earnings per share:
Basic		$ 3.35	$ 2.45	$ 2.04
Diluted		$ 3.27	$ 2.40	$ 2.00
Weighted-average common shares outstanding:
Basic		18,766,000	18,989,000	19,335,000
Diluted		19,244,000	19,416,000	19,663,000
Other comprehensive income (loss), net of tax	[1]	$ 4,601	$ (2,149)	$ (5,010)
Comprehensive income, net of tax		$ 67,477	$ 44,435	$ 34,359

[1] Net of tax of $1.0 million, $2.2 million, and $1.0 million for the years ended December 31, 2017, 2016, and 2015, respectively.