Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 6.5	$ 1.1	$ 2.8
Other income, net	3.7	2.8	4.5
Total revenues	3,573.4	3,051.4	2,613.4
Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	2,377.7	2,010.0	1,751.4
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	911.2	833.0	674.0
Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	159.2	122.3	109.7
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	88.7	82.2	71.0
Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	26.4	0.0	0.0
Operating Segments | Retail
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0.1	0.3	0.2
Other income, net	2.6	1.0	1.3
Total revenues	2,084.3	1,767.9	1,472.8
Operating Segments | Retail | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1,420.9	1,198.1	1,054.6
Operating Segments | Retail | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	473.5	414.9	275.9
Operating Segments | Retail | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	138.4	114.7	105.0
Operating Segments | Retail | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	48.8	38.9	35.8
Operating Segments | Retail | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0	0.0
Operating Segments | National Programs
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	1.3	0.6	0.8
Other income, net	0.1	0.2	0.1
Total revenues	859.5	701.9	610.6
Operating Segments | National Programs | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	590.2	488.7	422.9
Operating Segments | National Programs | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	198.5	173.8	159.2
Operating Segments | National Programs | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	15.4	3.3	0.3
Operating Segments | National Programs | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	27.6	35.3	27.3
Operating Segments | National Programs | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	26.4	0.0	0.0
Operating Segments | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0.3	0.2	0.2
Other income, net	0.3	0.6	0.4
Total revenues	453.4	403.4	352.8
Operating Segments | Wholesale Brokerage | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	366.6	323.1	273.9
Operating Segments | Wholesale Brokerage | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	68.5	67.2	66.0
Operating Segments | Wholesale Brokerage | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	5.4	4.3	4.4
Operating Segments | Wholesale Brokerage | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	12.3	8.0	7.9
Operating Segments | Wholesale Brokerage | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0	0.0
Operating Segments | Services
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0.0	0.0	0.0
Other income, net	0.0	0.0	0.0
Total revenues	171.9	178.9	174.0
Operating Segments | Services | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0	0.0
Operating Segments | Services | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	171.9	178.9	174.0
Operating Segments | Services | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	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
Operating Segments | Services | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0	0.0
Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	4.8	0.0	1.6
Other income, net	0.7	1.0	2.7
Total revenues	4.3	(0.7)	3.2
Corporate, Non-Segment | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.1	0.0
Corporate, Non-Segment | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	(1.2)	(1.8)	(1.1)
Corporate, Non-Segment | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	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
Corporate, Non-Segment | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	$ 0.0	$ 0.0	$ 0.0