Revenues - Summary of Impact of Adoption of Accounting Standards (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 1,614,762	$ 1,625,687	$ 1,586,008
Direct salaries and related costs	1,042,289	1,072,907	1,039,677
Income from operations	89,800	63,202	87,042
Income before income taxes	85,923	56,917	81,307
Income taxes	21,842	7,991	49,091
Net income	$ 64,081	$ 48,926	$ 32,216
Net income per common share:
Basic	$ 1.54	$ 1.16	$ 0.77
Diluted	$ 1.53	$ 1.16	$ 0.76
Balances Without the Impact of the ASC 606 Adoption [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues		$ 1,608,731
Direct salaries and related costs		1,069,667
Income from operations		49,486
Income before income taxes		43,201
Income taxes		4,833
Net income		$ 38,368
Net income per common share:
Basic		$ 0.91
Diluted		$ 0.91
Effect of Adoption Increase (Decrease) [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues		$ 16,956
Direct salaries and related costs		3,240
Income from operations		13,716
Income before income taxes		13,716
Income taxes		3,158
Net income		$ 10,558
Net income per common share:
Basic		$ 0.25
Diluted		$ 0.25