Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total revenues	$ 727,352	$ 598,806	$ 1,542,628	$ 1,297,302
Investment income	229	333	479	1,495
Amortization	29,546	26,741	59,039	53,132
Depreciation	8,801	6,168	16,257	12,189
Interest expense	16,285	13,809	32,626	29,100
Income before income taxes	186,309	129,401	425,515	334,655
Total assets	9,357,895	8,201,058	9,357,895	8,201,058	$ 8,966,492
Capital expenditures	13,717	19,480	25,142	35,938
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	401,099	312,562	924,014	760,051
Investment income	17	41	35	124
Amortization	19,002	16,453	37,835	32,740
Depreciation	2,805	2,147	5,563	4,183
Interest expense	22,604	21,303	45,224	43,101
Income before income taxes	70,481	36,829	221,704	165,491
Total assets	7,393,285	6,487,850	7,393,285	6,487,850
Capital expenditures	1,371	3,228	3,687	5,727
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	176,274	154,667	331,167	283,080
Investment income	134	129	286	392
Amortization	6,862	6,866	13,737	13,231
Depreciation	2,364	2,093	4,511	3,998
Interest expense	2,869	5,195	6,997	9,877
Income before income taxes	66,716	48,344	107,864	77,990
Total assets	3,570,836	3,398,709	3,570,836	3,398,709
Capital expenditures	3,652	1,532	6,653	3,045
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	104,591	88,853	195,466	166,551
Investment income	44	46	85	97
Amortization	2,364	2,032	4,761	4,381
Depreciation	680	467	1,324	898
Interest expense	4,051	2,594	8,304	4,305
Income before income taxes	26,392	24,708	45,173	42,394
Total assets	1,844,315	1,574,931	1,844,315	1,574,931
Capital expenditures	612	555	1,101	1,782
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	44,887	42,930	91,861	87,382
Investment income	0	0	3	0
Amortization	1,318	1,390	2,706	2,780
Depreciation	364	353	745	704
Interest expense	718	1,042	1,539	2,133
Income before income taxes	7,665	6,436	16,922	16,516
Total assets	447,265	464,905	447,265	464,905
Capital expenditures	352	206	509	473
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues	501	(206)	120	238
Investment income	34	117	70	882
Amortization	0	0	0	0
Depreciation	2,588	1,108	4,114	2,406
Interest expense	(13,957)	(16,325)	(29,438)	(30,316)
Income before income taxes	15,055	13,084	33,852	32,264
Total assets	(3,897,806)	(3,725,337)	(3,897,806)	(3,725,337)
Capital expenditures	$ 7,730	$ 13,959	$ 13,192	$ 24,911