Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 0.4	$ 0.2	$ 0.6	$ 0.5
Other income, net	0.6	1.2	0.8	2.2
Total revenues	839.7	727.3	1,744.5	1,542.6
Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	556.9	473.0	1,146.0	1,012.2
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	234.2	213.3	440.9	397.6
Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	20.2	20.0	99.1	84.6
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	22.1	19.6	50.7	45.5
Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	5.3	0.0	6.4	0.0
Operating Segments | Retail
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0.0	0.0	0.0	0.0
Other income, net	0.4	0.1	0.5	0.8
Total revenues	457.6	401.1	1,054.0	924.0
Operating Segments | Retail | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	301.6	263.6	695.1	620.9
Operating Segments | Retail | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	126.2	110.3	238.8	197.5
Operating Segments | Retail | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	20.0	18.7	92.2	80.7
Operating Segments | Retail | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	9.4	8.4	27.4	24.1
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	0.2	0.1	0.3	0.3
Other income, net	0.0	0.0	0.0	0.2
Total revenues	225.7	176.3	387.9	331.2
Operating Segments | National Programs | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	160.2	125.4	277.5	236.0
Operating Segments | National Programs | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	46.9	41.2	81.5	75.6
Operating Segments | National Programs | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	2.8	0.6	3.9	1.8
Operating Segments | National Programs | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	10.3	9.0	18.3	17.3
Operating Segments | National Programs | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	5.3	0.0	6.4	0.0
Operating Segments | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0.0	0.0	0.1	0.1
Other income, net	0.1	0.1	0.2	0.2
Total revenues	112.4	104.6	215.3	195.4
Operating Segments | Wholesale Brokerage | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	95.1	84.0	173.4	155.3
Operating Segments | Wholesale Brokerage | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	17.4	17.6	33.6	33.6
Operating Segments | Wholesale Brokerage | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	(2.6)	0.7	3.0	2.1
Operating Segments | Wholesale Brokerage | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	2.4	2.2	5.0	4.1
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	44.0	44.9	87.6	91.9
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	44.0	44.9	87.6	91.9
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	0.2	0.1	0.2	0.1
Other income, net	0.1	1.0	0.1	1.0
Total revenues	0.0	0.4	(0.3)	0.1
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	(0.3)	(0.7)	(0.6)	(1.0)
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