Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 10.3	$ 0.4	$ 17.3	$ 0.6
Other income, net	1.1	0.6	2.1	0.8
Total revenues	1,047.3	839.7	2,163.3	1,744.5
Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	708.9	556.9	1,455.6	1,146.0
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	251.9	234.2	489.9	440.9
Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	29.8	20.2	116.3	99.1
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	33.6	22.1	60.4	50.7
Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	11.7	5.3	21.7	6.4
Operating Segments | Retail
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0.3	0.0	0.5	0.0
Other income, net	0.9	0.4	1.4	0.5
Total revenues	574.5	457.6	1,288.0	1,054.0
Operating Segments | Retail | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	397.3	301.6	886.2	695.1
Operating Segments | Retail | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	133.7	126.2	259.8	238.8
Operating Segments | Retail | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	27.3	20.0	109.7	92.2
Operating Segments | Retail | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	15.0	9.4	30.4	27.4
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	2.0	0.2	3.2	0.3
Other income, net	0.1	0.0	0.3	0.0
Total revenues	283.3	225.7	513.1	387.9
Operating Segments | National Programs | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	197.3	160.2	355.1	277.5
Operating Segments | National Programs | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	55.3	46.9	105.6	81.5
Operating Segments | National Programs | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1.8	2.8	4.2	3.9
Operating Segments | National Programs | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	15.1	10.3	23.0	18.3
Operating Segments | National Programs | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	11.7	5.3	21.7	6.4
Operating Segments | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0.5	0.0	0.7	0.1
Other income, net	0.0	0.1	0.1	0.2
Total revenues	139.1	112.4	262.5	215.3
Operating Segments | Wholesale Brokerage | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	114.3	95.1	214.3	173.4
Operating Segments | Wholesale Brokerage | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	20.1	17.4	38.0	33.6
Operating Segments | Wholesale Brokerage | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.7	(2.6)	2.4	3.0
Operating Segments | Wholesale Brokerage | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	3.5	2.4	7.0	5.0
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	43.2	44.0	87.5	87.6
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	43.2	44.0	87.5	87.6
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
Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	7.5	0.2	12.9	0.2
Other income, net	0.1	0.1	0.3	0.1
Total revenues	7.2	0.0	12.2	(0.3)
Corporate, Non-Segment | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0	0.0	0.0
Corporate, Non-Segment | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	(0.4)	(0.3)	(1.0)	(0.6)
Corporate, Non-Segment | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0	0.0	0.0
Corporate, Non-Segment | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0	0.0	0.0
Corporate, Non-Segment | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	$ 0.0	$ 0.0	$ 0.0	$ 0.0