Quarterly Financial Data (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 28, 2012	Sep. 28, 2012	Jun. 29, 2012	Mar. 30, 2012	Dec. 30, 2011	Sep. 30, 2011	Jul. 01, 2011	Apr. 01, 2011	Dec. 28, 2012	Dec. 30, 2011	Dec. 31, 2010
Quarterly Financial Information [Line Items]
Revenues before reimbursements	$ 65,049	$ 66,725	$ 68,318	$ 66,470	$ 60,524	$ 61,387	$ 60,573	$ 64,183	$ 266,562	$ 246,667	$ 221,860
Revenues	72,946	73,298	74,484	71,925	67,916	65,951	65,106	73,473	292,653	272,446	248,753
Operating income	13,715	14,728	17,361	11,816	11,193	16,524	13,377	12,366	57,620	53,460	43,241
Income before income taxes	14,438	16,330	17,248	13,733	13,080	14,400	13,960	13,379	61,749	54,819	46,708
Net income	$ 8,472	$ 10,225	$ 10,327	$ 8,201	$ 7,731	$ 8,744	$ 8,217	$ 8,003	$ 37,225	$ 32,695	$ 27,521
Net income per share
Basic	$ 0.62	$ 0.75	$ 0.75	$ 0.59	$ 0.56	$ 0.63	$ 0.57	$ 0.55	$ 2.70	$ 2.31	$ 1.92
Diluted	$ 0.60	$ 0.72	$ 0.72	$ 0.57	$ 0.54	$ 0.60	$ 0.55	$ 0.53	$ 2.60	$ 2.22	$ 1.83
Shares used in per share computations
Basic	13,733	13,694	13,835	13,859	13,775	13,975	14,402	14,532	13,780	14,181	14,354
Diluted	14,208	14,196	14,316	14,419	14,322	14,554	14,971	15,140	14,293	14,751	15,069