Leases - Schedule of Right-of-Use Assets and Lease Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Jan. 01, 2019
Lessee, Lease, Description [Line Items]
ROU lease asset	$ 38.3	$ 22.6	$ 23.2
Lease liability	$ 39.1	$ 22.8	$ 23.2
Operating lease, weighted average incremental borrowing rate, percent	4.60%	4.60%
HG/BAM
Lessee, Lease, Description [Line Items]
ROU lease asset	$ 9.9	$ 10.4
Lease liability	9.9	10.4
Specialty Insurance Distribution (NSM)
Lessee, Lease, Description [Line Items]
ROU lease asset	17.0	4.8
Lease liability	17.0	4.8
Asset Management (Kudu)
Lessee, Lease, Description [Line Items]
ROU lease asset	2.1	2.3
Lease liability	2.1	2.3
Other Operations
Lessee, Lease, Description [Line Items]
ROU lease asset	9.3	5.1
Lease liability	$ 10.1	$ 5.3