Goodwill and Other Intangible Assets - Schedule of Acquisition Date Fair Values, Accumulated Amortization and Net Carrying Values for Other Intangibles Assets and Goodwill, By Company (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 439.2	$ 439.2	$ 168.8	$ 168.8	$ 168.8	$ 176.5
Other intangible assets, finite-lived, accumulated amortization	33.6		17.0
Total other intangible assets	320.1		218.8
Total other intangible assets	286.5	291.8	201.8	203.3	204.8	215.9
Total goodwill and other intangible assets	759.3		387.6
Goodwill and other intangible assets	725.7	$ 731.0	370.6	$ 372.1	$ 373.6	$ 392.4
Other
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, finite-lived, accumulated amortization	33.6		17.0
Total other intangible assets	286.5		201.8
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	292.5		292.5
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	116.8		116.8
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Underwriting Capacity
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, indefinite-lived	175.7		175.7
Operating Segments | Asset Management (Kudu)
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	7.6		7.6
Goodwill and other intangible assets	$ 8.0		8.3
Operating Segments | Asset Management (Kudu) | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	7 years
Other intangible assets, finite-lived, acquisition date fair value	$ 2.2		2.2
Other intangible assets, finite-lived, accumulated amortization	1.8		1.5
Other intangible assets, finite-lived, net carrying value	0.4		0.7
Operating Segments | P&C Insurance Distribution (Bamboo)
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	270.4		0.0
Other intangible assets, finite-lived, acquisition date fair value	101.0		0.0
Other intangible assets, finite-lived, accumulated amortization	12.4		0.0
Other intangible assets, finite-lived, net carrying value	88.6		0.0
Goodwill and other intangible assets	$ 359.0		0.0
Operating Segments | P&C Insurance Distribution (Bamboo) | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	10 years
Other intangible assets, finite-lived, acquisition date fair value	$ 23.5		0.0
Other intangible assets, finite-lived, accumulated amortization	1.8		0.0
Other intangible assets, finite-lived, net carrying value	$ 21.7		0.0
Operating Segments | P&C Insurance Distribution (Bamboo) | Agency relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	6 years
Other intangible assets, finite-lived, acquisition date fair value	$ 72.4		0.0
Other intangible assets, finite-lived, accumulated amortization	9.1		0.0
Other intangible assets, finite-lived, net carrying value	$ 63.3		0.0
Operating Segments | P&C Insurance Distribution (Bamboo) | Developed technology
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	3 years
Other intangible assets, finite-lived, acquisition date fair value	$ 4.7		0.0
Other intangible assets, finite-lived, accumulated amortization	1.1		0.0
Other intangible assets, finite-lived, net carrying value	$ 3.6		0.0
Operating Segments | P&C Insurance Distribution (Bamboo) | Other
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	3 months 18 days
Other intangible assets, finite-lived, acquisition date fair value	$ 0.4		0.0
Other intangible assets, finite-lived, accumulated amortization	0.4		0.0
Other intangible assets, finite-lived, net carrying value	0.0		0.0
Other Operations
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	44.4		44.4
Other intangible assets, finite-lived, acquisition date fair value	41.2		40.9
Other intangible assets, finite-lived, accumulated amortization	19.4		15.5
Other intangible assets, finite-lived, net carrying value	21.8		25.4
Goodwill and other intangible assets	$ 66.2		69.8
Other Operations | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	13 years 3 months 18 days
Other intangible assets, finite-lived, acquisition date fair value	$ 13.3		13.3
Other intangible assets, finite-lived, accumulated amortization	5.3		4.1
Other intangible assets, finite-lived, net carrying value	$ 8.0		9.2
Other Operations | Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	11 years
Other intangible assets, finite-lived, acquisition date fair value	$ 24.8		24.8
Other intangible assets, finite-lived, accumulated amortization	12.7		10.3
Other intangible assets, finite-lived, net carrying value	$ 12.1		14.5
Other Operations | Other
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	11 years 9 months 18 days
Other intangible assets, finite-lived, acquisition date fair value	$ 3.1		2.8
Other intangible assets, finite-lived, accumulated amortization	1.4		1.1
Other intangible assets, finite-lived, net carrying value	$ 1.7		$ 1.7