Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022
Segment Reporting Information [Line Items]
Total revenues	$ 1,116.0	$ 904.7
Investment income	7.0	0.2
Amortization	41.4	31.1
Depreciation	9.9	8.1
Interest expense	46.7	18.3
Income before income taxes	294.2	265.0
Total assets	13,399.9	11,272.9	$ 13,973.5
Capital expenditures	11.8	10.0
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	713.5	596.4
Investment income	0.2	0.0
Amortization	27.4	21.1
Depreciation	4.3	2.5
Interest expense	21.8	23.6
Income before income taxes	210.2	184.1
Total assets	7,605.3	4,903.9
Capital expenditures	4.8	1.6
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	229.8	162.2
Investment income	1.2	0.1
Amortization	10.0	6.7
Depreciation	3.0	2.8
Interest expense	9.8	2.2
Income before income taxes	73.3	41.7
Total assets	3,675.2	3,338.8
Capital expenditures	4.9	5.6
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	123.4	102.9
Investment income	0.2	0.1
Amortization	2.7	2.0
Depreciation	0.7	0.7
Interest expense	2.7	3.5
Income before income taxes	31.2	25.6
Total assets	1,440.7	1,124.8
Capital expenditures	0.4	0.4
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	44.3	43.6
Investment income	0.0	0.0
Amortization	1.3	1.3
Depreciation	0.4	0.4
Interest expense	0.4	0.6
Income before income taxes	5.2	6.6
Total assets	285.0	290.5
Capital expenditures	0.3	0.2
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues	5.0	(0.4)
Investment income	5.4	0.0
Amortization	0.0	0.0
Depreciation	1.5	1.7
Interest expense	12.0	(11.6)
Income before income taxes	(25.7)	7.0
Total assets	393.7	1,614.9
Capital expenditures	$ 1.4	$ 2.2