Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total revenues	$ 770,305	$ 673,962	$ 2,312,933	$ 1,971,264
Investment income	430	349	909	1,844
Amortization	29,523	27,059	88,562	80,190
Depreciation	9,200	6,647	25,457	18,836
Interest expense	16,175	13,234	48,802	42,334
Income before income taxes	196,489	158,528	622,004	493,183
Total assets	9,629,166	8,795,379	9,629,166	8,795,379	$ 8,966,492
Capital expenditures	9,475	19,882	34,617	55,820
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	423,421	359,473	1,347,435	1,119,524
Investment income	236	19	271	143
Amortization	19,057	16,624	56,892	49,363
Depreciation	2,775	2,347	8,337	6,530
Interest expense	22,417	20,519	67,641	63,620
Income before income taxes	71,639	56,057	293,342	221,549
Total assets	7,385,770	6,583,606	7,385,770	6,583,606
Capital expenditures	2,092	5,232	5,779	10,959
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	191,065	168,018	522,232	451,098
Investment income	136	205	423	597
Amortization	6,821	7,100	20,558	20,331
Depreciation	2,987	2,300	7,498	6,298
Interest expense	2,190	5,335	9,188	15,212
Income before income taxes	72,358	47,171	180,222	125,160
Total assets	3,789,376	3,530,345	3,789,376	3,530,345
Capital expenditures	4,660	2,203	11,313	5,248
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	112,468	101,239	307,934	267,790
Investment income	35	45	120	141
Amortization	2,360	1,945	7,121	6,326
Depreciation	649	548	1,973	1,446
Interest expense	3,916	2,488	12,220	6,793
Income before income taxes	29,365	35,038	74,539	77,432
Total assets	1,918,263	1,646,287	1,918,263	1,646,287
Capital expenditures	210	1,170	1,311	2,952
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	43,732	43,497	135,593	130,879
Investment income	0	0	3	0
Amortization	1,285	1,390	3,991	4,170
Depreciation	374	355	1,120	1,059
Interest expense	680	1,004	2,219	3,137
Income before income taxes	7,080	6,041	24,002	22,557
Total assets	449,494	467,889	449,494	467,889
Capital expenditures	887	584	1,396	1,057
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues	(381)	1,735	(261)	1,973
Investment income	23	80	92	963
Amortization	0	0	0	0
Depreciation	2,415	1,097	6,529	3,503
Interest expense	(13,028)	(16,112)	(42,466)	(46,428)
Income before income taxes	16,047	14,221	49,899	46,485
Total assets	(3,913,737)	(3,432,748)	(3,913,737)	(3,432,748)
Capital expenditures	$ 1,626	$ 10,693	$ 14,818	$ 35,604