Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total revenues	$ 3,573.4	$ 3,051.4	$ 2,613.4
Investment income	6.5	1.1	2.8
Amortization	146.6	119.6	108.5
Depreciation	39.2	33.3	26.3
Interest expense	141.2	65.0	59.0
Income before income taxes	876.1	762.8	624.1
Total assets	13,973.5	9,795.4	8,966.5
Capital expenditures	52.6	45.0	70.7
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	2,084.3	1,767.9	1,472.8
Investment income	0.1	0.3	0.2
Amortization	96.7	77.8	67.3
Depreciation	12.8	11.2	9.1
Interest expense	94.3	91.4	86.0
Income before income taxes	466.7	334.4	262.2
Total assets	7,458.6	5,040.7	7,093.6
Capital expenditures	18.6	8.1	13.2
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	859.5	701.9	610.6
Investment income	1.3	0.6	0.8
Amortization	35.4	27.4	27.2
Depreciation	15.3	9.8	8.7
Interest expense	33.0	11.4	20.6
Income before income taxes	271.1	242.3	182.9
Total assets	4,467.8	2,943.0	3,511.0
Capital expenditures	20.2	13.5	7.2
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	453.4	403.4	352.8
Investment income	0.3	0.2	0.2
Amortization	9.4	9.1	8.5
Depreciation	2.7	2.6	1.9
Interest expense	12.9	16.0	10.3
Income before income taxes	117.7	94.8	93.6
Total assets	1,401.6	1,154.4	1,791.7
Capital expenditures	2.8	1.6	3.3
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	171.9	178.9	174.0
Investment income	0.0	0.0	0.0
Amortization	5.1	5.3	5.5
Depreciation	1.6	1.5	1.4
Interest expense	2.1	2.9	4.1
Income before income taxes	24.1	28.3	28.0
Total assets	295.0	299.2	480.4
Capital expenditures	1.0	1.6	1.4
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues	4.3	(0.7)	3.2
Investment income	4.8	0.0	1.6
Amortization	0.0
Depreciation	6.8	8.2	5.2
Interest expense	(1.1)	(56.7)	(62.0)
Income before income taxes	(3.5)	63.0	57.4
Total assets	350.5	358.1	(3,910.2)
Capital expenditures	$ 10.0	$ 20.2	$ 45.6