Quarterly Financial Data (unaudited) (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 504,993	$ 513,012	$ 512,098	$ 497,774	$ 467,711	$ 448,962	$ 444,715	$ 446,344	$ 2,027,877	$ 1,807,732	$ 1,810,394
Operating expenses
Direct cost of revenues	340,162	336,477	330,318	321,117	307,566	294,851	304,071	309,072	1,328,074	1,215,560	1,210,771
Selling, general and administrative expenses	118,163	117,448	117,897	112,128	112,043	104,161	108,119	107,690	465,636	432,013	436,716
Special charges					10,811	0	30,074	0
Amortization of other intangible assets	1,865	1,975	2,052	2,270	2,766	2,882	2,422	2,493	8,162	10,563	10,306
Costs and Expenses	460,190	455,900	450,267	435,515	433,186	401,894	444,686	419,255	1,801,872	1,699,021	1,668,238
Operating income	44,803	57,112	61,831	62,259	34,525	47,068	29	27,089	226,005	108,711	142,156
Interest income and other	2,903	1,400	2,474	(1,800)	452	1,103	1,592	605	4,977	3,752	10,466
Interest expense	(7,076)	(7,246)	(6,583)	(6,244)	(6,547)	(6,760)	(6,250)	(5,801)	(27,149)	(25,358)	(24,819)
Gain on sale of business	0	13,031	0	0					13,031	0	0
Loss on early extinguishment of debt	(9,072)	0	0	0					(9,072)	0	0
Income before income tax provision	31,558	64,297	57,722	54,215	28,430	41,411	(4,629)	21,893	207,792	87,105	127,803
Income tax provision	7,834	19,964	14,113	15,270	(38,458)	9,197	527	7,877	57,181	(20,857)	42,283
Net income	$ 23,724	$ 44,333	$ 43,609	$ 38,945	$ 66,888	$ 32,214	$ (5,156)	$ 14,016	$ 150,611	$ 107,962	$ 85,520
Earnings per common share — basic (in dollars per share)	$ 0.63	$ 1.19	$ 1.18	$ 1.06	$ 1.81	$ 0.86	$ (0.13)	$ 0.35	$ 4.06	$ 2.79	$ 2.09
Earnings per common share — diluted (in dollars per share)	$ 0.61	$ 1.14	$ 1.14	$ 1.04	$ 1.78	$ 0.85	$ (0.13)	$ 0.34	$ 3.93	$ 2.75	$ 2.05
Weighted average common shares outstanding
Basic (in shares)	37,368	37,318	37,001	36,700	36,906	37,431	39,555	40,527	37,098	38,697	40,943
Diluted (in shares)	38,628	38,756	38,271	37,612	37,643	37,746	39,555	41,245	38,318	39,192	41,709