Quarterly Results of Operations - Summary of Quarterly Results of Operations (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 251,105	$ 251,661	$ 254,226	$ 239,688	$ 236,864	$ 228,608	$ 228,423	$ 219,469	$ 996,680	$ 913,364	$ 826,990
Cost of services	77,455	78,876	76,816	75,427	72,256	68,151	69,696	65,300	308,574	275,403	230,282
Selling, general and administrative	69,345	70,833	71,516	67,658	64,174	59,917	52,842	55,515	279,352	232,448	211,905
Amortization of intangible assets	11,591	11,574	11,442	11,270	11,217	11,193	11,222	11,166	45,877	44,798	50,017
Depreciation and amortization of property, equipment and leasehold improvements	7,620	6,342	5,921	5,828	5,570	5,443	4,774	4,597	25,930	22,302	18,700
Total operating expenses	166,011	167,625	165,695	160,183	153,217	144,704	138,534	136,578	659,514	573,033	508,755
Operating income	85,094	84,036	88,531	79,505	83,647	83,904	89,889	82,891	337,166	340,331	318,235
Interest income	(226)	(277)	(192)	(156)	(239)	(227)	(186)	(237)	(851)	(889)	(699)
Interest expense	15,791	5,604	5,366	5,059	6,913	5,828	6,499	7,016	31,820	26,256	56,401
Other expense (income)	(1,199)	(1,287)	(726)	1,071	(21)	563	(328)	1,922	(2,141)	2,136	1,732
Other expense (income), net	14,366	4,040	4,448	5,974	6,653	6,164	5,985	8,701	28,828	27,503	57,434
Income from continuing operations before provision for income taxes	70,728	79,996	84,083	73,531	76,994	77,740	83,904	74,190	308,338	312,828	260,801
Provision for income taxes	27,459	28,272	27,280	26,385	36,119	27,804	27,763	21,232	109,396	112,918	96,010
Income from continuing operations	43,269	51,724	56,803	47,146	40,875	49,936	56,141	52,958	198,942	199,910	164,791
Income (loss) from discontinued operations, net of income taxes	1,071	(10)	50,857	33,253	6,382	5,374	4,912	5,979	85,171	22,647	19,447
Net income	$ 44,340	$ 51,714	$ 107,660	$ 80,399	$ 47,257	$ 55,310	$ 61,053	$ 58,937	$ 284,113	$ 222,557	$ 184,238
Earnings per basic common share
From continuing operations	$ 0.38	$ 0.44	$ 0.48	$ 0.40	$ 0.34	$ 0.42	$ 0.46	$ 0.44	$ 1.72	$ 1.66	$ 1.34
From discontinued operations	$ 0.01		$ 0.44	$ 0.28	$ 0.06	$ 0.04	$ 0.04	$ 0.05
Earnings per basic common share	$ 0.39	$ 0.44	$ 0.92	$ 0.68	$ 0.40	$ 0.46	$ 0.50	$ 0.49	$ 2.45	$ 1.85	$ 1.50
Earnings per diluted common share
From continuing operations	$ 0.38	$ 0.44	$ 0.48	$ 0.40	$ 0.34	$ 0.42	$ 0.46	$ 0.43	$ 1.70	$ 1.64	$ 1.32
From discontinued operations	$ 0.01		$ 0.43	$ 0.28	$ 0.05	$ 0.04	$ 0.04	$ 0.05
Earnings per diluted common share	$ 0.39	$ 0.44	$ 0.91	$ 0.68	$ 0.39	$ 0.46	$ 0.50	$ 0.48	$ 2.43	$ 1.83	$ 1.48
Weighted average shares outstanding used in computing per share data
Basic	112,299	116,251	116,702	117,582	118,828	119,607	121,149	120,746	115,737	120,100	122,023
Diluted	113,289	117,163	117,664	118,597	119,877	120,578	122,069	121,702	116,706	121,074	123,204