Segment Information - Summarized Financial Information Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Total revenues	$ 404,597	$ 432,167	$ 419,447	$ 404,298	$ 393,020	$ 421,418	$ 397,764	$ 363,594	$ 1,660,509	$ 1,575,796	$ 1,363,279
Investment income									1,004	747	638
Amortization									87,421	82,941	67,932
Depreciation									20,890	20,895	17,485
Interest expense									39,248	28,408	16,440
Income before income taxes	95,037	$ 112,830	$ 100,914	$ 93,778	38,446	$ 112,685	$ 101,781	$ 86,837	402,559	339,749	357,609
Total assets	5,012,739				4,956,458				5,012,739	4,956,458	3,649,508
Capital expenditures									18,375	24,923	16,366
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues									870,346	823,686	737,349
Investment income									87	67	82
Amortization									45,145	42,935	38,523
Depreciation									6,558	6,449	5,874
Interest expense									41,036	43,502	34,658
Income before income taxes									181,938	157,491	161,787
Total assets	3,507,476				3,229,484				3,507,476	3,229,484	3,012,688
Capital expenditures									6,797	6,873	6,886
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues									428,734	404,239	301,372
Investment income									210	164	19
Amortization									28,479	25,129	14,953
Depreciation									7,250	7,805	5,492
Interest expense									55,705	49,663	24,014
Income before income taxes									67,673	73,178	61,223
Total assets	2,505,752				2,455,749				2,505,752	2,455,749	1,377,404
Capital expenditures									6,001	14,133	4,810
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues									216,996	211,911	193,710
Investment income									150	26	22
Amortization									9,739	10,703	10,719
Depreciation									2,142	2,470	2,674
Interest expense									891	1,294	2,316
Income before income taxes									64,708	8,276	47,501
Total assets	895,782				857,804				895,782	857,804	865,731
Capital expenditures									3,084	1,526	1,825
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues									145,365	136,558	131,489
Investment income									42	3	1
Amortization									4,019	4,135	3,698
Depreciation									1,988	2,213	1,623
Interest expense									5,970	7,678	7,322
Income before income taxes									19,713	17,870	25,791
Total assets	285,459				296,034				285,459	296,034	277,652
Capital expenditures									1,088	1,210	1,811
Other
Segment Reporting Information [Line Items]
Total revenues									(932)	(598)	(641)
Investment income									515	487	514
Amortization									39	39	39
Depreciation									2,952	1,958	1,822
Interest expense									(64,354)	(73,729)	(51,870)
Income before income taxes									68,527	82,934	61,307
Total assets	$ (2,181,730)				$ (1,882,613)				(2,181,730)	(1,882,613)	(1,883,967)
Capital expenditures									$ 1,405	$ 1,181	$ 1,034