Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Segment Reporting Information [Line Items]
Total revenues	$ 1,047.3	$ 839.7	$ 2,163.3	$ 1,744.5
Investment income	10.3	0.4	17.3	0.6
Amortization	41.2	33.6	82.6	64.7
Depreciation	10.2	8.9	20.1	17.1
Interest expense	47.9	36.0	94.6	54.3
Income before income taxes	254.4	198.8	548.6	463.9
Total assets	14,071.5	12,279.9	14,071.5	12,279.9	$ 13,973.5
Capital expenditures	13.3	8.3	25.1	18.3
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	574.5	457.6	1,288.0	1,054.0
Investment income	0.3	0.0	0.5	0.0
Amortization	27.2	20.5	54.6	41.6
Depreciation	4.6	2.6	8.9	5.2
Interest expense	21.8	23.5	43.6	47.1
Income before income taxes	105.1	82.5	315.4	266.5
Total assets	7,867.5	5,036.3	7,867.5	5,036.3
Capital expenditures	9.3	2.1	14.1	3.7
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	283.3	225.7	513.1	387.9
Investment income	2.0	0.2	3.2	0.3
Amortization	10.1	9.8	20.1	16.5
Depreciation	2.9	3.5	5.9	6.3
Interest expense	8.8	10.4	18.6	12.6
Income before income taxes	118.9	76.5	192.2	118.1
Total assets	4,025.4	3,554.9	4,025.4	3,554.9
Capital expenditures	2.6	5.3	7.5	11.0
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	139.1	112.4	262.5	215.3
Investment income	0.5	0.0	0.7	0.1
Amortization	2.6	2.0	5.3	4.0
Depreciation	0.7	0.6	1.4	1.3
Interest expense	2.8	3.3	5.5	6.8
Income before income taxes	37.5	33.8	68.7	59.5
Total assets	1,422.5	1,149.0	1,422.5	1,149.0
Capital expenditures	1.0	0.4	1.4	0.8
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	43.2	44.0	87.5	87.6
Investment income	0.0	0.0	0.0	0.0
Amortization	1.3	1.3	2.6	2.6
Depreciation	0.4	0.4	0.8	0.8
Interest expense	0.3	0.5	0.7	1.1
Income before income taxes	5.7	6.6	10.9	13.2
Total assets	296.4	287.9	296.4	287.9
Capital expenditures	0.4	0.3	0.7	0.5
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues	7.2	0.0	12.2	(0.3)
Investment income	7.5	0.2	12.9	0.2
Amortization	0.0	0.0	0.0	0.0
Depreciation	1.6	1.8	3.1	3.5
Interest expense	14.2	(1.7)	26.2	(13.3)
Income before income taxes	(12.8)	(0.6)	(38.6)	6.6
Total assets	(459.7)	2,251.8	(459.7)	2,251.8
Capital expenditures	$ 0.0	$ 0.2	$ 1.4	$ 2.3