Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Total revenues	$ 4,257.1	$ 3,573.4	$ 3,051.4
Investment income	52.4	6.5	1.1
Amortization	166.0	146.6	119.6
Depreciation	40.0	39.2	33.3
Interest expense	190.0	141.2	65.0
Income before income taxes	1,146.1	876.1	762.8
Total assets	14,883.4	13,973.5	9,795.4
Capital expenditures	68.9	52.6	45.0
Capital expenditures	68.9	52.6	45.0
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	2,440.0	2,084.3	1,767.9
Investment income	1.3	0.1	0.3
Amortization	108.5	96.7	77.8
Depreciation	18.0	12.8	11.2
Interest expense	83.4	94.3	91.4
Income before income taxes	528.4	466.7	334.4
Total assets	8,303.9	7,458.6	5,040.7
Capital expenditures	45.5	18.6	8.1
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	1,077.3	859.5	701.9
Investment income	12.1	1.3	0.6
Amortization	41.2	35.4	27.4
Depreciation	11.8	15.3	9.8
Interest expense	35.6	33.0	11.4
Income before income taxes	405.5	271.1	242.3
Total assets	4,333.5	4,467.8	2,943.0
Capital expenditures	16.5	20.2	13.5
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	540.7	453.4	403.4
Investment income	1.5	0.3	0.2
Amortization	11.2	9.4	9.1
Depreciation	2.6	2.7	2.6
Interest expense	11.9	12.9	16.0
Income before income taxes	125.8	117.7	94.8
Total assets	1,558.9	1,401.6	1,154.4
Capital expenditures	3.0	2.8	1.6
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	163.1	171.9	178.9
Investment income	0.0	0.0	0.0
Amortization	5.1	5.1	5.3
Depreciation	1.4	1.6	1.5
Interest expense	1.3	2.1	2.9
Income before income taxes	153.8	24.1	28.3
Total assets	209.0	295.0	299.2
Capital expenditures	1.1	1.0	1.6
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues	36.0	4.3	(0.7)
Investment income	37.5	4.8	0.0
Amortization	0.0
Depreciation	6.2	6.8	8.2
Interest expense	(57.8)	(1.1)	(56.7)
Income before income taxes	(67.4)	(3.5)	63.0
Total assets	478.1	350.5	358.1
Capital expenditures	$ 2.8	$ 10.0	$ 20.2