Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total revenues	$ 673,962	$ 618,683	$ 1,971,264	$ 1,813,182
Investment income	349	1,668	1,844	4,274
Amortization	27,059	26,272	80,190	78,418
Depreciation	6,647	5,815	18,836	17,516
Interest expense	13,234	16,314	42,334	47,805
Income before income taxes	158,528	151,838	493,183	423,880
Total assets	8,795,379	7,680,664	8,795,379	7,680,664	$ 7,622,821
Capital expenditures	19,882	12,184	55,820	47,358
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	359,473	337,712	1,119,524	1,037,681
Investment income	19	28	143	45
Amortization	16,624	15,821	49,363	46,622
Depreciation	2,347	1,897	6,530	5,440
Interest expense	20,519	22,287	63,620	64,641
Income before income taxes	56,057	51,133	221,549	189,191
Total assets	6,583,606	6,155,670	6,583,606	6,155,670
Capital expenditures	5,232	2,266	10,959	7,810
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	168,018	142,854	451,098	384,231
Investment income	205	375	597	1,071
Amortization	7,100	6,264	20,331	19,217
Depreciation	2,300	1,779	6,298	4,990
Interest expense	5,335	3,557	15,212	13,134
Income before income taxes	47,171	46,629	125,160	106,188
Total assets	3,530,345	3,315,656	3,530,345	3,315,656
Capital expenditures	2,203	1,276	5,248	7,786
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	101,239	87,109	267,790	238,789
Investment income	45	48	141	131
Amortization	1,945	2,797	6,326	8,490
Depreciation	548	422	1,446	1,251
Interest expense	2,488	1,105	6,793	3,692
Income before income taxes	35,038	28,908	77,432	67,421
Total assets	1,646,287	1,406,709	1,646,287	1,406,709
Capital expenditures	1,170	768	2,952	2,470
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	43,497	50,106	130,879	150,395
Investment income	0	35	0	117
Amortization	1,390	1,390	4,170	4,089
Depreciation	355	315	1,059	902
Interest expense	1,004	1,164	3,137	3,239
Income before income taxes	6,041	14,775	22,557	33,220
Total assets	467,889	469,292	467,889	469,292
Capital expenditures	584	209	1,057	540
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues	1,735	902	1,973	2,086
Investment income	80	1,182	963	2,910
Amortization	0	0	0	0
Depreciation	1,097	1,402	3,503	4,933
Interest expense	(16,112)	(11,799)	(46,428)	(36,901)
Income before income taxes	14,221	10,393	46,485	27,860
Total assets	(3,432,748)	(3,666,663)	(3,432,748)	(3,666,663)
Capital expenditures	$ 10,693	$ 7,665	$ 35,604	$ 28,752