Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total revenues	$ 815,276	$ 698,495
Investment income	250	1,161
Amortization	29,493	26,391
Depreciation	7,456	6,021
Interest expense	16,342	15,291
Income before income taxes	239,207	205,253
Total assets	8,910,793	7,582,345	$ 8,966,492
Capital expenditures	11,425	16,458
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	522,915	447,489
Investment income	18	83
Amortization	18,833	16,287
Depreciation	2,758	2,036
Interest expense	22,621	21,798
Income before income taxes	151,223	128,662
Total assets	7,210,499	6,375,532
Capital expenditures	2,316	2,499
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	154,893	128,412
Investment income	152	263
Amortization	6,875	6,365
Depreciation	2,147	1,905
Interest expense	4,128	4,682
Income before income taxes	41,148	29,645
Total assets	3,398,682	3,054,411
Capital expenditures	3,001	1,513
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	90,875	77,698
Investment income	41	50
Amortization	2,397	2,349
Depreciation	644	431
Interest expense	4,253	1,711
Income before income taxes	18,782	17,686
Total assets	1,787,033	1,514,243
Capital expenditures	489	1,227
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	46,974	44,452
Investment income	3	0
Amortization	1,388	1,390
Depreciation	381	351
Interest expense	821	1,091
Income before income taxes	9,257	10,080
Total assets	483,920	475,168
Capital expenditures	157	267
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues	(381)	444
Investment income	36	765
Amortization	0	0
Depreciation	1,526	1,298
Interest expense	(15,481)	(13,991)
Income before income taxes	18,797	19,180
Total assets	(3,969,341)	(3,837,009)
Capital expenditures	$ 5,462	$ 10,952