Quarterly Financial Data (unaudited) (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 602,218	$ 593,106	$ 606,119	$ 551,274	$ 504,993	$ 513,012	$ 512,098	$ 497,774	$ 2,352,717	$ 2,027,877	$ 1,807,732
Operating expenses
Direct cost of revenues	418,672	380,892	386,266	349,066	340,162	336,477	330,318	321,117	1,534,896	1,328,074	1,215,560
Selling, general and administrative expenses	133,032	127,951	129,906	113,185	118,163	117,448	117,897	112,128	504,074	465,636	432,013
Amortization of other intangible assets	2,314	2,125	1,852	1,861	1,865	1,975	2,052	2,270	8,152	8,162	10,563
Costs and Expenses	554,018	510,968	518,024	464,112	460,190	455,900	450,267	435,515	2,047,122	1,801,872	1,699,021
Operating income	48,200	82,138	88,095	87,162	44,803	57,112	61,831	62,259	305,595	226,005	108,711
Interest income and other	(3,680)	2,973	2,609	159	2,903	1,400	2,474	(1,800)	2,061	4,977	3,752
Interest expense	(4,835)	(4,832)	(4,793)	(4,746)	(7,076)	(7,246)	(6,583)	(6,244)	(19,206)	(27,149)	(25,358)
Gain on sale of business					0	13,031	0	0	0	13,031	0
Loss on early extinguishment of debt					(9,072)	0	0	0	0	(9,072)	0
Income before income tax provision	39,685	80,279	85,911	82,575	31,558	64,297	57,722	54,215	288,450	207,792	87,105
Income tax provision	10,624	19,857	21,313	19,930	7,834	19,964	14,113	15,270	71,724	57,181	(20,857)
Net income	$ 29,061	$ 60,422	$ 64,598	$ 62,645	$ 23,724	$ 44,333	$ 43,609	$ 38,945	$ 216,726	$ 150,611	$ 107,962
Earnings per common share — basic (in dollars per share)	$ 0.80	$ 1.65	$ 1.75	$ 1.69	$ 0.63	$ 1.19	$ 1.18	$ 1.06	$ 5.89	$ 4.06	$ 2.79
Earnings per common share — diluted (in dollars per share)	$ 0.76	$ 1.59	$ 1.69	$ 1.64	$ 0.61	$ 1.14	$ 1.14	$ 1.04	$ 5.69	$ 3.93	$ 2.75
Weighted average common shares outstanding
Basic (in shares)	36,545	36,617	36,960	36,981	37,368	37,318	37,001	36,700	36,774	37,098	38,697
Diluted (in shares)	38,126	37,938	38,168	38,219	38,628	38,756	38,271	37,612	38,111	38,318	39,192