Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Segment Reporting Information [Line Items]
Total revenues	$ 1,067.7	$ 927.6	$ 3,230.9	$ 2,672.0
Investment income	16.6	1.2	33.8	1.8
Amortization	41.1	43.5	123.7	108.2
Depreciation	10.4	11.3	30.5	28.3
Interest expense	47.5	41.5	142.1	95.8
Income before income taxes	242.3	218.0	791.0	681.8
Total assets	13,983.0	13,658.3	13,983.0	13,658.3	$ 13,973.5
Capital expenditures	13.8	14.1	38.4	32.4
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	586.0	528.2	1,874.2	1,582.3
Investment income	0.5	0.0	1.0	0.0
Amortization	27.0	28.2	81.6	69.8
Depreciation	4.9	3.9	13.8	9.1
Interest expense	21.3	22.8	64.9	69.9
Income before income taxes	110.8	112.2	426.2	378.8
Total assets	7,762.1	7,128.1	7,762.1	7,128.1
Capital expenditures	8.9	4.7	22.7	8.4
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	278.2	231.4	791.2	619.3
Investment income	3.8	0.3	7.0	0.6
Amortization	10.1	11.4	30.2	27.9
Depreciation	2.9	4.6	8.8	10.9
Interest expense	8.6	10.2	27.3	22.8
Income before income taxes	111.7	69.7	303.8	187.9
Total assets	3,942.8	4,476.3	3,942.8	4,476.3
Capital expenditures	3.3	3.3	10.8	14.2
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	149.6	126.3	412.0	341.6
Investment income	0.6	0.0	1.2	0.2
Amortization	2.7	2.6	8.0	6.7
Depreciation	0.7	0.7	2.1	2.0
Interest expense	3.1	3.2	8.6	10.0
Income before income taxes	23.8	35.4	92.4	94.8
Total assets	1,489.6	1,366.5	1,489.6	1,366.5
Capital expenditures	0.7	0.7	2.1	1.5
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	42.4	41.1	129.9	128.8
Investment income	0.0	0.0	0.0	0.0
Amortization	1.3	1.3	3.9	3.9
Depreciation	0.3	0.4	1.2	1.2
Interest expense	0.3	0.5	1.0	1.6
Income before income taxes	4.8	4.7	15.8	18.0
Total assets	279.8	289.1	279.8	289.1
Capital expenditures	0.2	0.3	0.9	0.8
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues	11.5	0.6	23.6	0.0
Investment income	11.7	0.8	24.6	1.0
Amortization	0.0	0.0	0.0	0.0
Depreciation	1.6	1.7	4.6	5.1
Interest expense	14.2	(4.8)	40.3	(8.5)
Income before income taxes	(8.8)	(4.0)	(47.2)	2.3
Total assets	508.7	398.3	508.7	398.3
Capital expenditures	$ 0.7	$ 5.1	$ 1.9	$ 7.5