Segment Information - Summarized Financial Information Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total revenues	$ 462,274	$ 432,167	$ 1,332,965	$ 1,255,912
Investment income	230	228	1,150	708
Amortization	21,805	22,158	65,025	65,406
Depreciation	5,195	5,215	15,867	15,635
Interest expense	9,883	9,882	29,617	29,404
Income before income taxes	116,972	112,830	328,598	307,522
Total assets	5,522,027	5,002,192	5,522,027	5,002,192	$ 5,004,479
Capital expenditures	4,191	4,176	13,135	12,773
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	228,645	216,249	695,393	657,314
Investment income	5	19	33	62
Amortization	10,861	11,552	32,743	33,671
Depreciation	1,508	1,632	4,761	4,908
Interest expense	9,026	10,121	29,415	30,841
Income before income taxes	44,894	41,785	144,496	137,249
Total assets	3,652,977	3,400,407	3,652,977	3,400,407
Capital expenditures	1,443	1,637	4,664	4,410
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	123,632	116,966	333,522	319,577
Investment income	96	49	583	150
Amortization	6,921	7,133	21,011	21,343
Depreciation	1,945	1,850	5,881	5,372
Interest expense	10,844	13,398	34,895	42,306
Income before income taxes	32,319	25,281	68,367	48,567
Total assets	2,933,568	2,542,619	2,933,568	2,542,619
Capital expenditures	2,153	1,754	5,399	5,004
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	70,192	61,374	184,893	168,619
Investment income	0	3	4	148
Amortization	2,882	2,441	7,915	7,296
Depreciation	503	506	1,487	1,630
Interest expense	1,540	200	2,472	645
Income before income taxes	20,862	22,276	51,711	53,150
Total assets	1,053,516	880,454	1,053,516	880,454
Capital expenditures	11	425	925	2,087
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	39,586	37,980	117,906	111,128
Investment income	57	0	204	1
Amortization	1,140	1,022	3,345	3,067
Depreciation	473	474	1,432	1,533
Interest expense	1,257	1,527	3,820	4,722
Income before income taxes	5,971	6,022	17,929	16,062
Total assets	342,360	296,659	342,360	296,659
Capital expenditures	80	233	561	774
Other
Segment Reporting Information [Line Items]
Total revenues	219	(402)	1,251	(726)
Investment income	72	157	326	347
Amortization	1	10	11	29
Depreciation	766	753	2,306	2,192
Interest expense	(12,784)	(15,364)	(40,985)	(49,110)
Income before income taxes	12,926	17,466	46,095	52,494
Total assets	(2,460,394)	(2,117,947)	(2,460,394)	(2,117,947)
Capital expenditures	$ 504	$ 127	$ 1,586	$ 498