Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 16.6	$ 1.2	$ 33.8	$ 1.8
Other income, net	1.8	1.2	3.9	2.0
Total revenues	1,067.7	927.6	3,230.9	2,672.0
Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	721.0	630.1	2,176.7	1,775.9
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	257.8	246.1	747.8	686.8
Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	31.1	29.6	147.3	128.9
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	27.3	7.8	87.7	58.5
Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	12.1	11.6	33.7	18.1
Operating Segments | Retail
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0.5	0.0	1.0	0.0
Other income, net	1.6	1.1	3.1	1.6
Total revenues	586.0	528.2	1,874.2	1,582.3
Operating Segments | Retail | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	411.2	360.6	1,297.3	1,055.7
Operating Segments | Retail | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	133.8	130.6	393.8	369.4
Operating Segments | Retail | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	29.2	25.0	138.9	117.3
Operating Segments | Retail | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	9.7	10.9	40.1	38.3
Operating Segments | Retail | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0	0.0	0.0
Operating Segments | National Programs
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	3.8	0.3	7.0	0.6
Other income, net	0.2	0.0	0.4	0.0
Total revenues	278.2	231.4	791.2	619.3
Operating Segments | National Programs | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	188.2	165.8	543.6	443.2
Operating Segments | National Programs | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	60.8	56.2	166.4	137.6
Operating Segments | National Programs | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1.0	3.7	5.1	7.7
Operating Segments | National Programs | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	12.1	(6.2)	35.0	12.0
Operating Segments | National Programs | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	12.1	11.6	33.7	18.1
Operating Segments | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0.6	0.0	1.2	0.2
Other income, net	0.0	0.1	0.1	0.2
Total revenues	149.6	126.3	412.0	341.6
Operating Segments | Wholesale Brokerage | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	121.5	103.7	335.7	277.1
Operating Segments | Wholesale Brokerage | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	21.1	18.4	59.1	52.0
Operating Segments | Wholesale Brokerage | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.9	0.9	3.3	3.9
Operating Segments | Wholesale Brokerage | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	5.5	3.1	12.6	8.2
Operating Segments | Wholesale Brokerage | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0	0.0	0.0
Operating Segments | Services
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0.0	0.0	0.0	0.0
Other income, net	0.0	0.0	0.0	0.0
Total revenues	42.4	41.1	129.9	128.8
Operating Segments | Services | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0	0.0	0.0
Operating Segments | Services | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	42.4	41.1	129.9	128.8
Operating Segments | Services | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0	0.0	0.0
Operating Segments | Services | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0	0.0	0.0
Operating Segments | Services | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0	0.0	0.0
Other
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	11.7	0.8	24.6	1.0
Other income, net	0.0	0.0	0.3	0.1
Total revenues	11.5	0.6	23.6	0.0
Other | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.1	0.0	0.1	(0.1)
Other | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	(0.3)	(0.2)	(1.4)	(1.0)
Other | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0	0.0	0.0
Other | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0	0.0	0.0
Other | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	$ 0.0	$ 0.0	$ 0.0	$ 0.0