Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 1.2	$ 0.4	$ 1.8	$ 0.9
Other income, net	1.2	0.2	2.0	2.4
Total revenues	927.6	770.3	2,672.0	2,312.9
Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	630.1	509.2	1,775.9	1,521.4
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	246.1	220.9	686.8	618.4
Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	29.6	22.0	128.9	106.6
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	7.8	17.6	58.5	63.2
Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	11.6	0.0	18.1	0.0
Operating Segments | Retail
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0.0	0.2	0.0	0.3
Other income, net	1.1	0.1	1.6	0.9
Total revenues	528.2	423.4	1,582.3	1,347.4
Operating Segments | Retail | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	360.6	284.6	1,055.7	905.5
Operating Segments | Retail | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	130.6	109.9	369.4	307.3
Operating Segments | Retail | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	25.0	19.9	117.3	100.6
Operating Segments | Retail | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	10.9	8.7	38.3	32.8
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	0.3	0.1	0.6	0.4
Other income, net	0.0	0.1	0.1	0.2
Total revenues	231.4	191.1	619.3	522.3
Operating Segments | National Programs | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	165.8	133.6	443.2	369.7
Operating Segments | National Programs | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	56.2	49.9	137.6	125.3
Operating Segments | National Programs | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	3.7	0.9	7.7	2.8
Operating Segments | National Programs | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	(6.2)	6.5	12.0	23.9
Operating Segments | National Programs | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	11.6	0.0	18.1	0.0
Operating Segments | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0.1	0.0	0.2	0.1
Other income, net	0.1	0.0	0.2	0.3
Total revenues	126.3	112.5	341.6	307.9
Operating Segments | Wholesale Brokerage | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	103.7	91.0	277.1	246.2
Operating Segments | Wholesale Brokerage | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	18.4	17.9	52.0	51.6
Operating Segments | Wholesale Brokerage | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.9	1.2	3.9	3.2
Operating Segments | Wholesale Brokerage | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	3.1	2.4	8.2	6.5
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	41.1	43.7	128.8	135.6
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	41.1	43.7	128.8	135.6
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	0.8	0.1	1.0	0.1
Other income, net	0.0	0.0	0.1	1.0
Total revenues	0.6	(0.4)	0.0	(0.3)
Corporate, Non-Segment | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0.0	0.0	(0.1)	0.0
Corporate, Non-Segment | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	(0.2)	(0.5)	(1.0)	(1.4)
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