Segment Information - Summarized Financial Information Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Total revenues	$ 433,664	$ 462,274	$ 446,518	$ 424,173	$ 404,597	$ 432,167	$ 419,447	$ 404,298	$ 1,766,629	$ 1,660,509	$ 1,575,796
Investment income									1,456	1,004	747
Amortization									86,663	87,421	82,941
Depreciation									21,003	20,890	20,895
Interest expense									39,481	39,248	28,408
Income before income taxes	94,901	$ 116,972	$ 109,077	$ 102,549	95,037	$ 112,830	$ 100,914	$ 93,778	423,499	402,559	339,749
Total assets	5,287,343				5,004,479				5,287,343	5,004,479	4,946,560
Capital expenditures									17,765	18,375	24,923
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues									917,406	870,346	823,686
Investment income									37	87	67
Amortization									43,447	45,145	42,935
Depreciation									6,191	6,558	6,449
Interest expense									38,216	41,036	43,502
Income before income taxes									188,001	181,938	157,491
Total assets	3,854,393				3,507,476				3,854,393	3,507,476	3,229,484
Capital expenditures									5,951	6,797	6,873
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues									448,516	428,734	404,239
Investment income									628	210	164
Amortization									27,920	28,479	25,129
Depreciation									7,868	7,250	7,805
Interest expense									45,738	55,705	49,663
Income before income taxes									91,762	67,673	73,178
Total assets	2,711,378				2,505,752				2,711,378	2,505,752	2,455,749
Capital expenditures									6,977	6,001	14,133
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues									243,103	216,996	211,911
Investment income									4	150	26
Amortization									10,801	9,739	10,703
Depreciation									1,975	2,142	2,470
Interest expense									3,976	891	1,294
Income before income taxes									62,623	64,708	8,276
Total assets	1,108,829				895,782				1,108,829	895,782	857,804
Capital expenditures									1,301	3,084	1,526
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues									156,365	145,365	136,558
Investment income									283	42	3
Amortization									4,485	4,019	4,135
Depreciation									1,881	1,988	2,213
Interest expense									4,950	5,970	7,678
Income before income taxes									24,338	19,713	17,870
Total assets	371,645				285,459				371,645	285,459	296,034
Capital expenditures									656	1,088	1,210
Other
Segment Reporting Information [Line Items]
Total revenues									1,239	(932)	(598)
Investment income									504	515	487
Amortization									10	39	39
Depreciation									3,088	2,952	1,958
Interest expense									(53,399)	(64,354)	(73,729)
Income before income taxes									56,775	68,527	82,934
Total assets	$ (2,758,902)				$ (2,189,990)				(2,758,902)	(2,189,990)	(1,892,511)
Capital expenditures									$ 2,880	$ 1,405	$ 1,181