Comparative Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 28, 2018	Sep. 28, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 29, 2017		Sep. 29, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 28, 2018		Dec. 29, 2017		Dec. 30, 2016
Quarterly Financial Information [Line Items]
Revenues before reimbursements	$ 85,269	$ 88,714	$ 89,972	$ 90,684	$ 82,718		$ 82,359	$ 84,120	$ 80,467	$ 354,639		$ 329,664		$ 299,197
Revenues	92,143	95,302	95,621	96,457	88,282		87,555	87,840	84,122	379,523	[1]	347,799	[1]	315,076	[1]
Operating income	26,786	20,594	22,478	21,598	17,795		19,305	20,317	14,634	91,456		72,051		61,911
Income before income taxes	21,959	23,989	24,919	22,450	20,721		22,030	22,348	17,410	93,317		82,509		69,122
Net income (loss)	$ 16,036	$ 17,453	$ 18,425	$ 20,340	$ (3,705)	[2]	$ 14,643	$ 13,791	$ 16,576	$ 72,254		$ 41,305		$ 47,480
Net income per share
Basic	$ 0.30	$ 0.33	$ 0.35	$ 0.39	$ (0.07)		$ 0.28	$ 0.26	$ 0.32	$ 1.37		$ 0.78		$ 0.90
Diluted	$ 0.30	$ 0.32	$ 0.34	$ 0.38	$ (0.07)		$ 0.27	$ 0.26	$ 0.31	$ 1.33		$ 0.77		$ 0.87
Shares used in per share computations
Basic	52,839	53,032	53,008	52,744	52,726		52,740	52,830	52,604	52,906		52,724		52,976
Diluted	54,119	54,302	54,195	54,012	52,726		53,926	53,936	53,962	54,168		53,972		54,332

[1] Geographic revenues are allocated based on the location of the client.

[2] The decrease in net income and diluted earnings per share during the fourth quarter of 2017 was due to the impact of the tax legislation. During the fourth quarter of 2017, the Company recorded a tax expense of $16,507,000 related to the tax legislation signed into law during the fourth quarter of 2017. The Company has domestic deferred tax assets primarily associated with its deferred compensation plan and stock-based compensation program, which were previously valued at the federal corporate income tax rate of 35%. The Company’s deferred tax assets were re-measured at the lower enacted corporate tax rate of 21% which contributed $15,137,000 to the increase in income tax associated with the tax legislation. The Company also has foreign earnings that were subject to the mandatory repatriation tax. The total mandatory repatriation tax, net of the benefit of its foreign tax credits, contributed $1,370,000 to the increase in income tax expense associated with the tax legislation.