Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Segment Reporting Information [Line Items]
Total revenues	$ 927.6	$ 770.3	$ 2,672.0	$ 2,312.9
Investment income	1.2	0.4	1.8	0.9
Amortization	43.5	29.5	108.2	88.6
Depreciation	11.3	9.2	28.3	25.4
Interest expense	41.5	16.2	95.8	48.8
Income before income taxes	218.0	196.5	681.8	622.0
Total assets	13,658.3	9,629.2	13,658.3	9,629.2	$ 9,795.4
Capital expenditures	14.1	9.5	32.4	34.6
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	528.2	423.4	1,582.3	1,347.4
Investment income	0.0	0.2	0.0	0.3
Amortization	28.2	19.1	69.8	56.9
Depreciation	3.9	2.8	9.1	8.3
Interest expense	22.8	22.4	69.9	67.6
Income before income taxes	112.2	71.6	378.8	293.4
Total assets	7,128.1	7,385.8	7,128.1	7,385.8
Capital expenditures	4.7	2.1	8.4	5.8
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	231.4	191.1	619.3	522.3
Investment income	0.3	0.1	0.6	0.4
Amortization	11.4	6.8	27.9	20.6
Depreciation	4.6	3.0	10.9	7.5
Interest expense	10.2	2.2	22.8	9.2
Income before income taxes	69.7	72.4	187.9	180.2
Total assets	4,476.3	3,789.4	4,476.3	3,789.4
Capital expenditures	3.3	4.7	14.2	11.3
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	126.3	112.5	341.6	307.9
Investment income	0.1	0.0	0.2	0.1
Amortization	2.6	2.3	6.7	7.1
Depreciation	0.7	0.6	2.0	2.0
Interest expense	3.2	3.9	10.0	12.2
Income before income taxes	35.4	29.4	94.8	74.5
Total assets	1,366.5	1,918.2	1,366.5	1,918.2
Capital expenditures	0.7	0.2	1.5	1.3
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	41.1	43.7	128.8	135.6
Investment income	0.0	0.0	0.0	0.0
Amortization	1.3	1.3	3.9	4.0
Depreciation	0.4	0.3	1.2	1.1
Interest expense	0.5	0.7	1.6	2.2
Income before income taxes	4.7	7.1	18.0	24.0
Total assets	289.1	449.5	289.1	449.5
Capital expenditures	0.3	0.9	0.8	1.4
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues	0.6	(0.4)		(0.3)
Investment income	0.8	0.1	1.0	0.1
Amortization			(0.1)
Depreciation	1.7	2.5	5.1	6.5
Interest expense	4.8	(13.0)	(8.5)	(42.4)
Income before income taxes	(4.0)	16.0	2.3	49.9
Total assets	398.3	(3,913.7)	398.3	(3,913.7)
Capital expenditures	$ 5.1	$ 1.6	$ 7.5	$ 14.8