Segment Information - Summarized Financial Information Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Total revenues	$ 432,167	$ 421,418	$ 1,255,912	$ 1,182,776
Investment income	228	225	708	522
Amortization	22,158	21,983	65,406	60,482
Depreciation	5,215	5,456	15,635	15,338
Interest expense	9,882	7,298	29,404	18,374
Income before income taxes	112,830	112,685	307,522	301,303
Total assets	5,002,192	4,945,748	5,002,192	4,945,748	$ 4,956,458
Capital expenditures	4,176	7,612	12,773	20,189
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	216,249	206,504	657,314	626,194
Investment income	19	16	62	48
Amortization	11,552	10,920	33,671	31,812
Depreciation	1,632	1,632	4,908	4,850
Interest expense	10,121	10,882	30,841	33,000
Income before income taxes	41,785	39,823	137,249	129,213
Total assets	3,400,407	3,163,014	3,400,407	3,163,014
Capital expenditures	1,637	1,320	4,410	5,016
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	116,966	117,926	319,577	288,087
Investment income	49	48	150	119
Amortization	7,133	7,343	21,343	17,512
Depreciation	1,850	2,143	5,372	5,503
Interest expense	13,398	15,915	42,306	33,803
Income before income taxes	25,281	20,694	48,567	47,640
Total assets	2,542,619	2,508,815	2,542,619	2,508,815
Capital expenditures	1,754	5,243	5,004	12,369
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	61,374	61,620	168,619	165,894
Investment income	3	9	148	20
Amortization	2,441	2,676	7,296	8,028
Depreciation	506	640	1,630	1,890
Interest expense	200	299	645	1,060
Income before income taxes	22,276	21,750	53,150	47,863
Total assets	880,454	895,943	880,454	895,943
Capital expenditures	425	492	2,087	1,205
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	37,980	35,492	111,128	102,891
Investment income	0	0	1	2
Amortization	1,022	1,034	3,067	3,101
Depreciation	474	555	1,533	1,646
Interest expense	1,527	1,937	4,722	5,878
Income before income taxes	6,022	5,145	16,062	12,977
Total assets	296,659	285,307	296,659	285,307
Capital expenditures	233	325	774	860
Other
Segment Reporting Information [Line Items]
Total revenues	(402)	(124)	(726)	(290)
Investment income	157	152	347	333
Amortization	10	10	29	29
Depreciation	753	486	2,192	1,449
Interest expense	(15,364)	(21,735)	(49,110)	(55,367)
Income before income taxes	17,466	25,273	52,494	63,610
Total assets	(2,117,947)	(1,907,331)	(2,117,947)	(1,907,331)
Capital expenditures	$ 127	$ 232	$ 498	$ 739