Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 0.2	$ 0.3
Other income, net	0.2	1.0
Total revenues	904.7	815.3
Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	589.1	539.5
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	206.6	184.1
Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	78.9	64.5
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	28.6	25.9
Earned Premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1.1	0.0
Operating Segments | Retail
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0.0	0.0
Other income, net	0.1	0.7
Total revenues	596.4	522.9
Operating Segments | Retail | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	393.5	357.5
Operating Segments | Retail | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	112.7	87.1
Operating Segments | Retail | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	72.2	61.9
Operating Segments | Retail | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	17.9	15.7
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	0.1	0.2
Other income, net	0.0	0.1
Total revenues	162.2	154.9
Operating Segments | National Programs | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	117.3	110.7
Operating Segments | National Programs | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	34.6	34.4
Operating Segments | National Programs | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1.1	1.2
Operating Segments | National Programs | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	8.0	8.3
Operating Segments | National Programs | Earned Premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1.1	0.0
Operating Segments | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0.1	0.1
Other income, net	0.1	0.1
Total revenues	102.9	90.9
Operating Segments | Wholesale Brokerage | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	78.3	71.3
Operating Segments | Wholesale Brokerage | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	16.1	16.1
Operating Segments | Wholesale Brokerage | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	5.6	1.4
Operating Segments | Wholesale Brokerage | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	2.7	1.9
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	43.6	47.0
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	43.6	47.0
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	0.0	0.0
Other income, net	0.0	0.1
Total revenues	(0.4)	(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	(0.4)	(0.5)
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