Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 7.0	$ 0.2
Other income, net	1.0	0.2
Total revenues	1,116.0	904.7
Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	746.7	589.1
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	238.0	206.6
Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	86.5	78.9
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	26.8	28.6
Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	10.0	1.1
Operating Segments | Retail
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0.2	0.0
Other income, net	0.5	0.1
Total revenues	713.5	596.4
Operating Segments | Retail | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	488.9	393.5
Operating Segments | Retail | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	126.1	112.7
Operating Segments | Retail | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	82.4	72.2
Operating Segments | Retail | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	15.4	17.9
Operating Segments | Retail | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0
Operating Segments | National Programs
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	1.2	0.1
Other income, net	0.2	0.0
Total revenues	229.8	162.2
Operating Segments | National Programs | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	157.8	117.3
Operating Segments | National Programs | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	50.3	34.6
Operating Segments | National Programs | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	2.4	1.1
Operating Segments | National Programs | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	7.9	8.0
Operating Segments | National Programs | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	10.0	1.1
Operating Segments | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0.2	0.1
Other income, net	0.1	0.1
Total revenues	123.4	102.9
Operating Segments | Wholesale Brokerage | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	100.0	78.3
Operating Segments | Wholesale Brokerage | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	17.9	16.1
Operating Segments | Wholesale Brokerage | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1.7	5.6
Operating Segments | Wholesale Brokerage | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	3.5	2.7
Operating Segments | Wholesale Brokerage | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0
Operating Segments | Services
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0.0	0.0
Other income, net	0.0	0.0
Total revenues	44.3	43.6
Operating Segments | Services | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0
Operating Segments | Services | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	44.3	43.6
Operating Segments | Services | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	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
Operating Segments | Services | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0
Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	5.4	0.0
Other income, net	0.2	0.0
Total revenues	5.0	(0.4)
Corporate, Non-Segment | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0
Corporate, Non-Segment | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	(0.6)	(0.4)
Corporate, Non-Segment | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	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
Corporate, Non-Segment | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	$ 0.0	$ 0.0