Goodwill and Other Intangible Assets - Summary of Goodwill and Intangible Assets (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Intangible Asset, Acquired, Finite-Lived [Line Items]
Goodwill	$ 1,092.3	$ 534.6	$ 439.2	$ 439.2	$ 439.2	$ 168.8
Other intangible assets, finite-lived, accumulated amortization	57.1		39.0
Total other intangible assets	357.7		320.1
Total other intangible assets	300.6	270.8	281.1	286.5	291.8	201.8
Total goodwill and other intangible assets	1,450.0		759.3
Total goodwill and other intangible assets	1,392.9	$ 805.4	720.3	$ 725.7	$ 731.0	$ 370.6
Other
Intangible Asset, Acquired, Finite-Lived [Line Items]
Other intangible assets, finite-lived, accumulated amortization	57.1		39.0
Total other intangible assets	300.6		281.1
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Intangible Asset, Acquired, Finite-Lived [Line Items]
Total goodwill and other intangible assets	292.5		292.5
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark
Intangible Asset, Acquired, Finite-Lived [Line Items]
Goodwill	116.8		116.8
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Underwriting Capacity
Intangible Asset, Acquired, Finite-Lived [Line Items]
Other intangible assets, indefinite-lived	175.7		175.7
Operating Segments | Asset Management (Kudu)
Intangible Asset, Acquired, Finite-Lived [Line Items]
Goodwill	7.6		7.6
Total goodwill and other intangible assets	$ 7.7		8.0
Operating Segments | Asset Management (Kudu) | Trade names
Intangible Asset, Acquired, Finite-Lived [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	2.1		1.8
Other intangible assets, finite-lived, net carrying value	0.1		0.4
Operating Segments | P&C Insurance Distribution (Bamboo)
Intangible Asset, Acquired, Finite-Lived [Line Items]
Goodwill	270.4		270.4
Other intangible assets, finite-lived, acquisition date fair value	101.0		101.0
Other intangible assets, finite-lived, accumulated amortization	28.4		16.4
Other intangible assets, finite-lived, net carrying value	$ 72.6		84.6
Operating Segments | P&C Insurance Distribution (Bamboo) | Trade names
Intangible Asset, Acquired, Finite-Lived [Line Items]
Other intangible assets, weighted average useful life (in years)	10 years
Other intangible assets, finite-lived, acquisition date fair value	$ 23.5		23.5
Other intangible assets, finite-lived, accumulated amortization	4.1		2.3
Other intangible assets, finite-lived, net carrying value	$ 19.4		21.2
Operating Segments | P&C Insurance Distribution (Bamboo) | Agency relationships
Intangible Asset, Acquired, Finite-Lived [Line Items]
Other intangible assets, weighted average useful life (in years)	6 years
Other intangible assets, finite-lived, acquisition date fair value	$ 72.4		72.4
Other intangible assets, finite-lived, accumulated amortization	21.1		12.1
Other intangible assets, finite-lived, net carrying value	$ 51.3		60.3
Operating Segments | P&C Insurance Distribution (Bamboo) | Developed technology
Intangible Asset, Acquired, Finite-Lived [Line Items]
Other intangible assets, weighted average useful life (in years)	3 years
Other intangible assets, finite-lived, acquisition date fair value	$ 4.7		4.7
Other intangible assets, finite-lived, accumulated amortization	2.8		1.6
Other intangible assets, finite-lived, net carrying value	$ 1.9		3.1
Operating Segments | P&C Insurance Distribution (Bamboo) | Other
Intangible Asset, Acquired, Finite-Lived [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		0.4
Other intangible assets, finite-lived, accumulated amortization	0.4		0.4
Other intangible assets, finite-lived, net carrying value	0.0		0.0
Operating Segments | Specialty Insurance Distribution (Distinguished)
Intangible Asset, Acquired, Finite-Lived [Line Items]
Goodwill	595.4		0.0
Total goodwill and other intangible assets	595.4		0.0
Other Operations
Intangible Asset, Acquired, Finite-Lived [Line Items]
Goodwill	102.1		44.4
Other intangible assets, finite-lived, acquisition date fair value	78.8		41.2
Other intangible assets, finite-lived, accumulated amortization	26.6		20.8
Other intangible assets, finite-lived, net carrying value	52.2		20.4
Total goodwill and other intangible assets	$ 154.3		64.8
Other Operations | Trade names
Intangible Asset, Acquired, Finite-Lived [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	$ 22.4		13.3
Other intangible assets, finite-lived, accumulated amortization	7.1		5.6
Other intangible assets, finite-lived, net carrying value	$ 15.3		7.7
Other Operations | Customer relationships
Intangible Asset, Acquired, Finite-Lived [Line Items]
Other intangible assets, weighted average useful life (in years)	8 years 9 months 18 days
Other intangible assets, finite-lived, acquisition date fair value	$ 53.3		24.8
Other intangible assets, finite-lived, accumulated amortization	17.7		13.6
Other intangible assets, finite-lived, net carrying value	$ 35.6		11.2
Other Operations | Other
Intangible Asset, Acquired, Finite-Lived [Line Items]
Other intangible assets, weighted average useful life (in years)	11 years 8 months 12 days
Other intangible assets, finite-lived, acquisition date fair value	$ 3.1		3.1
Other intangible assets, finite-lived, accumulated amortization	1.8		1.6
Other intangible assets, finite-lived, net carrying value	$ 1.3		$ 1.5