Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
Segment Reporting Information [Line Items]
Total revenues	$ 839.7	$ 727.3	$ 1,744.5	$ 1,542.6
Investment income	0.4	0.2	0.6	0.5
Amortization	33.6	29.5	64.7	59.0
Depreciation	8.9	8.8	17.1	16.3
Interest expense	36.0	16.3	54.3	32.6
Income before income taxes	198.8	186.3	463.9	425.5
Total assets	12,279.9	9,357.9	12,279.9	9,357.9	$ 9,795.4
Capital expenditures	8.3	13.7	18.3	25.1
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	457.6	401.1	1,054.0	924.0
Investment income	0.0	0.0	0.0	0.0
Amortization	20.5	19.0	41.6	37.8
Depreciation	2.6	2.8	5.2	5.6
Interest expense	23.5	22.6	47.1	45.2
Income before income taxes	82.5	70.5	266.5	221.6
Total assets	5,036.3	7,393.3	5,036.3	7,393.3
Capital expenditures	2.1	1.4	3.7	3.7
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	225.7	176.3	387.9	331.2
Investment income	0.2	0.1	0.3	0.3
Amortization	9.8	6.9	16.5	13.7
Depreciation	3.5	2.3	6.3	4.5
Interest expense	10.4	2.9	12.6	7.0
Income before income taxes	76.5	66.7	118.1	107.9
Total assets	3,554.9	3,570.8	3,554.9	3,570.8
Capital expenditures	5.3	3.6	11.0	6.6
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	112.4	104.6	215.3	195.4
Investment income	0.0	0.0	0.1	0.1
Amortization	2.0	2.4	4.0	4.8
Depreciation	0.6	0.7	1.3	1.3
Interest expense	3.3	4.1	6.8	8.3
Income before income taxes	33.8	26.4	59.5	45.1
Total assets	1,149.0	1,844.3	1,149.0	1,844.3
Capital expenditures	0.4	0.6	0.8	1.1
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	44.0	44.9	87.6	91.9
Investment income	0.0	0.0	0.0	0.0
Amortization	1.3	1.3	2.6	2.7
Depreciation	0.4	0.4	0.8	0.7
Interest expense	0.5	0.7	1.1	1.5
Income before income taxes	6.6	7.7	13.2	16.9
Total assets	287.9	447.3	287.9	447.3
Capital expenditures	0.3	0.4	0.5	0.5
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues		0.4	(0.3)	0.1
Investment income	0.2	0.1	(0.2)	(0.1)
Amortization		(0.1)
Depreciation	1.8	2.6	3.5	4.2
Interest expense	(1.7)	(14.0)	(13.3)	(29.4)
Income before income taxes	(0.6)	15.0	6.6	34.0
Total assets	2,251.8	(3,897.8)	2,251.8	(3,897.8)
Capital expenditures	$ 0.2	$ 7.7	$ 2.3	$ 13.2