Segment Information - Summarized Financial Information Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total revenues	$ 474,316	$ 475,646	$ 466,305	$ 465,080	$ 433,664	$ 462,274	$ 446,518	$ 424,173	$ 1,881,347	$ 1,766,629	$ 1,660,509
Investment income									1,626	1,456	1,004
Amortization									85,446	86,663	87,421
Depreciation									22,698	21,003	20,890
Interest expense									38,316	39,481	39,248
Income before income taxes	106,334	$ 124,419	$ 108,002	$ 110,967	94,901	$ 116,972	$ 109,077	$ 102,549	449,722	423,499	402,559
Total assets	5,747,550				5,262,734				5,747,550	5,262,734	4,979,844
Capital expenditures									24,192	17,765	18,375
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues									943,460	917,406	870,346
Investment income									8	37	87
Amortization									42,164	43,447	45,145
Depreciation									5,210	6,191	6,558
Interest expense									31,133	38,216	41,036
Income before income taxes									196,616	188,001	181,938
Total assets	4,255,515				3,854,393				4,255,515	3,854,393	3,507,476
Capital expenditures									4,494	5,951	6,797
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues									479,813	448,516	428,734
Investment income									384	628	210
Amortization									27,277	27,920	28,479
Depreciation									6,325	7,868	7,250
Interest expense									35,561	45,738	55,705
Income before income taxes									109,961	91,762	67,673
Total assets	3,267,486				2,711,378				3,267,486	2,711,378	2,503,537
Capital expenditures									5,936	6,977	6,001
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues									271,737	243,103	216,996
Investment income									0	4	150
Amortization									11,456	10,801	9,739
Depreciation									1,885	1,975	2,142
Interest expense									6,263	3,976	891
Income before income taxes									68,844	62,623	64,708
Total assets	1,260,239				1,108,829				1,260,239	1,108,829	895,782
Capital expenditures									1,836	1,301	3,084
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues									165,372	156,365	145,365
Investment income									299	283	42
Amortization									4,548	4,485	4,019
Depreciation									1,600	1,881	1,988
Interest expense									3,522	4,950	5,970
Income before income taxes									30,498	24,338	19,713
Total assets	399,240				371,645				399,240	371,645	285,459
Capital expenditures									1,033	656	1,088
Other
Segment Reporting Information [Line Items]
Total revenues									20,965	1,239	(932)
Investment income									935	504	515
Amortization									1	10	39
Depreciation									7,678	3,088	2,952
Interest expense									(38,163)	(53,399)	(64,354)
Income before income taxes									43,803	56,775	68,527
Total assets	$ (3,434,930)				$ (2,783,511)				(3,434,930)	(2,783,511)	(2,212,410)
Capital expenditures									$ 10,893	$ 2,880	$ 1,405