Goodwill and Other Intangible Assets - Summary of Goodwill and Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Intangible Asset, Acquired, Finite-Lived [Line Items]
Acquisition Date Fair Value	$ 623.2
Goodwill, impairment loss	9.1
Goodwill	614.1	$ 439.2	$ 168.8
Other intangible assets, indefinite-lived	175.7
Other intangible assets, finite-lived, accumulated amortization	38.9	39.0
Impairments	0.5
Other intangible assets, finite-lived, net carrying value	230.4
Total other intangible assets	445.5	320.1
Total other intangible assets	406.1	281.1	201.8
Goodwill and intangible asset impairment	9.6
Total goodwill and other intangible assets	1,068.7	759.3
Total goodwill and other intangible assets	$ 1,020.2	720.3	$ 370.6
Goodwill, Impairment Loss, Statement of Income or Comprehensive Income [Extensible Enumeration]	General and administrative expenses
Other Operations
Intangible Asset, Acquired, Finite-Lived [Line Items]
Acquisition Date Fair Value	$ 102.1
Goodwill, impairment loss	9.1
Goodwill	93.0	44.4
Other intangible assets, finite-lived, acquisition date fair value	78.8	41.2
Other intangible assets, finite-lived, accumulated amortization	29.0	20.8
Impairments	0.5
Other intangible assets, finite-lived, net carrying value	49.3	20.4
Total goodwill and other intangible assets	142.3	64.8
Ark | Operating Segments
Intangible Asset, Acquired, Finite-Lived [Line Items]
Total goodwill and other intangible assets	292.5	292.5
Ark | Operating Segments | Ark
Intangible Asset, Acquired, Finite-Lived [Line Items]
Acquisition Date Fair Value	116.8
Goodwill, impairment loss	0.0
Goodwill	116.8	116.8
Ark | Operating Segments | Ark | Underwriting Capacity
Intangible Asset, Acquired, Finite-Lived [Line Items]
Other intangible assets, indefinite-lived	175.7	175.7
Kudu | Operating Segments
Intangible Asset, Acquired, Finite-Lived [Line Items]
Acquisition Date Fair Value	7.6
Goodwill, impairment loss	0.0
Goodwill	7.6	7.6
Total goodwill and other intangible assets	7.7	8.0
Bamboo | Operating Segments
Intangible Asset, Acquired, Finite-Lived [Line Items]
Acquisition Date Fair Value	0.0
Goodwill, impairment loss	0.0
Goodwill	0.0	270.4
Other intangible assets, finite-lived, acquisition date fair value	0.0	101.0
Other intangible assets, finite-lived, accumulated amortization	0.0	16.4
Impairments	0.0
Other intangible assets, finite-lived, net carrying value	0.0	84.6
Distinguished Partners | Operating Segments
Intangible Asset, Acquired, Finite-Lived [Line Items]
Acquisition Date Fair Value	396.7
Goodwill, impairment loss	0.0
Goodwill	396.7	0.0
Other intangible assets, finite-lived, acquisition date fair value	188.8	0.0
Other intangible assets, finite-lived, accumulated amortization	7.8	0.0
Impairments	0.0
Other intangible assets, finite-lived, net carrying value	181.0	0.0
Total goodwill and other intangible assets	$ 577.7	0.0
Trade names | Other Operations
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	$ 22.4	13.3
Other intangible assets, finite-lived, accumulated amortization	7.7	5.6
Impairments	0.2
Other intangible assets, finite-lived, net carrying value	$ 14.5	7.7
Trade names | Kudu | Operating Segments
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
Impairments	0.0
Other intangible assets, finite-lived, net carrying value	0.1	0.4
Trade names | Bamboo | Operating Segments
Intangible Asset, Acquired, Finite-Lived [Line Items]
Other intangible assets, finite-lived, acquisition date fair value	0.0	23.5
Other intangible assets, finite-lived, accumulated amortization	0.0	2.3
Impairments	0.0
Other intangible assets, finite-lived, net carrying value	$ 0.0	21.2
Trade names | Distinguished Partners | Operating Segments
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	$ 24.3	0.0
Other intangible assets, finite-lived, accumulated amortization	0.8	0.0
Impairments	0.0
Other intangible assets, finite-lived, net carrying value	23.5	0.0
Agency relationships | Bamboo | Operating Segments
Intangible Asset, Acquired, Finite-Lived [Line Items]
Other intangible assets, finite-lived, acquisition date fair value	0.0	72.4
Other intangible assets, finite-lived, accumulated amortization	0.0	12.1
Impairments	0.0
Other intangible assets, finite-lived, net carrying value	$ 0.0	60.3
Agency relationships | Distinguished Partners | Operating Segments
Intangible Asset, Acquired, Finite-Lived [Line Items]
Other intangible assets, weighted average useful life (in years)	8 years
Other intangible assets, finite-lived, acquisition date fair value	$ 36.6	0.0
Other intangible assets, finite-lived, accumulated amortization	2.0	0.0
Impairments	0.0
Other intangible assets, finite-lived, net carrying value	125.9	0.0
Developed technology | Bamboo | Operating Segments
Intangible Asset, Acquired, Finite-Lived [Line Items]
Other intangible assets, finite-lived, acquisition date fair value	0.0	4.7
Other intangible assets, finite-lived, accumulated amortization	0.0	1.6
Impairments	0.0
Other intangible assets, finite-lived, net carrying value	$ 0.0	3.1
Developed technology | Distinguished Partners | Operating Segments
Intangible Asset, Acquired, Finite-Lived [Line Items]
Other intangible assets, weighted average useful life (in years)	5 years
Other intangible assets, finite-lived, acquisition date fair value	$ 127.9	0.0
Other intangible assets, finite-lived, accumulated amortization	5.0	0.0
Impairments	0.0
Other intangible assets, finite-lived, net carrying value	$ 31.6	0.0
Customer relationships | Other Operations
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	19.4	13.6
Impairments	0.3
Other intangible assets, finite-lived, net carrying value	33.6	11.2
Total other intangible assets
Intangible Asset, Acquired, Finite-Lived [Line Items]
Other intangible assets, finite-lived, accumulated amortization	38.9	39.0
Impairments	0.5
Total other intangible assets	$ 406.1	281.1
Total other intangible assets | Other Operations
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	$ 3.1	3.1
Other intangible assets, finite-lived, accumulated amortization	1.9	1.6
Impairments	0.0
Other intangible assets, finite-lived, net carrying value	1.2	1.5
Total other intangible assets | Bamboo | Operating Segments
Intangible Asset, Acquired, Finite-Lived [Line Items]
Other intangible assets, finite-lived, acquisition date fair value	0.0	0.4
Other intangible assets, finite-lived, accumulated amortization	0.0	0.4
Impairments	0.0
Other intangible assets, finite-lived, net carrying value	$ 0.0	$ 0.0