Segment Information - Summarized Financial Information Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Total revenues	$ 419,447	$ 397,764	$ 823,745	$ 761,358
Investment income	260	194	480	297
Amortization	21,623	20,623	43,248	38,499
Depreciation	5,237	5,242	10,420	9,882
Interest expense	9,671	7,004	19,522	11,076
Income before income taxes	100,914	101,781	194,692	188,618
Total assets	5,020,837	4,930,472	5,020,837	4,930,472	$ 4,956,458
Capital expenditures	4,870	7,850	8,597	12,577
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	222,721	214,220	441,065	419,690
Investment income	21	16	43	32
Amortization	11,185	10,623	22,119	20,892
Depreciation	1,634	1,625	3,276	3,218
Interest expense	10,562	11,373	20,720	22,118
Income before income taxes	48,455	48,173	95,464	89,390
Total assets	3,423,263	3,203,327	3,423,263	3,203,327
Capital expenditures	1,349	1,580	2,773	3,696
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	103,056	93,486	202,611	170,161
Investment income	56	66	101	71
Amortization	6,975	6,305	14,210	10,169
Depreciation	1,756	1,871	3,522	3,360
Interest expense	13,953	12,446	28,908	17,888
Income before income taxes	13,810	10,002	23,286	26,946
Total assets	2,516,430	2,446,569	2,516,430	2,446,569
Capital expenditures	1,761	5,237	3,250	7,126
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	55,417	54,273	107,245	104,274
Investment income	73	7	145	11
Amortization	2,432	2,676	4,855	5,352
Depreciation	561	631	1,124	1,250
Interest expense	216	375	445	761
Income before income taxes	16,390	15,366	30,874	26,113
Total assets	865,000	913,500	865,000	913,500
Capital expenditures	1,211	426	1,662	713
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	38,360	35,757	73,148	67,399
Investment income	1	0	1	2
Amortization	1,022	1,010	2,045	2,067
Depreciation	529	628	1,059	1,091
Interest expense	1,596	1,971	3,195	3,941
Income before income taxes	5,538	5,063	10,040	7,832
Total assets	283,996	278,138	283,996	278,138
Capital expenditures	301	244	541	535
Other
Segment Reporting Information [Line Items]
Total revenues	(107)	28	(324)	(166)
Investment income	109	105	190	181
Amortization	9	9	19	19
Depreciation	757	487	1,439	963
Interest expense	(16,656)	(19,161)	(33,746)	(33,632)
Income before income taxes	16,721	23,177	35,028	38,337
Total assets	(2,067,852)	(1,911,062)	(2,067,852)	(1,911,062)
Capital expenditures	$ 248	$ 363	$ 371	$ 507