Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total revenues	$ 598,806	$ 575,219	$ 1,297,302	$ 1,194,499
Investment income	333	1,525	1,495	2,606
Amortization	26,741	25,954	53,132	52,146
Depreciation	6,168	5,666	12,189	11,701
Interest expense	13,809	16,293	29,100	31,491
Income before income taxes	129,401	123,522	334,655	272,042
Total assets	8,201,058	7,164,767	8,201,058	7,164,767	$ 7,622,821
Capital expenditures	19,480	11,988	35,938	35,174
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	312,562	310,738	760,051	699,969
Investment income	41	17	124	17
Amortization	16,453	15,357	32,740	30,801
Depreciation	2,147	1,919	4,183	3,543
Interest expense	21,303	21,201	43,101	42,354
Income before income taxes	36,829	44,217	165,491	138,058
Total assets	6,487,850	5,925,156	6,487,850	5,925,156
Capital expenditures	3,228	2,408	5,727	5,544
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	154,667	131,817	283,080	241,377
Investment income	129	380	392	696
Amortization	6,866	6,357	13,231	12,953
Depreciation	2,093	1,621	3,998	3,211
Interest expense	5,195	4,223	9,877	9,577
Income before income taxes	48,344	40,243	77,990	59,559
Total assets	3,398,709	2,981,933	3,398,709	2,981,933
Capital expenditures	1,532	1,942	3,045	6,510
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	88,853	81,171	166,551	151,680
Investment income	46	43	97	83
Amortization	2,032	2,844	4,381	5,693
Depreciation	467	425	898	829
Interest expense	2,594	1,260	4,305	2,587
Income before income taxes	24,708	22,944	42,394	38,513
Total assets	1,574,931	1,352,659	1,574,931	1,352,659
Capital expenditures	555	513	1,782	1,702
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	42,930	50,797	87,382	100,289
Investment income	0	34	0	82
Amortization	1,390	1,396	2,780	2,699
Depreciation	353	303	704	587
Interest expense	1,042	1,203	2,133	2,075
Income before income taxes	6,436	9,268	16,516	18,445
Total assets	464,905	466,162	464,905	466,162
Capital expenditures	206	222	473	331
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues	(206)	696	238	1,184
Investment income	117	1,051	882	1,728
Amortization	0	0	0	0
Depreciation	1,108	1,398	2,406	3,531
Interest expense	(16,325)	(11,594)	(30,316)	(25,102)
Income before income taxes	13,084	6,850	32,264	17,467
Total assets	(3,725,337)	(3,561,143)	(3,725,337)	(3,561,143)
Capital expenditures	$ 13,959	$ 6,903	$ 24,911	$ 21,087