Goodwill and Other Intangible Assets - Summary of Goodwill and Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Goodwill [Line Items]
Goodwill	$ 439.2	$ 168.8	$ 176.5
Indefinite-lived intangible assets	175.7
Other intangible assets, accumulated amortization	39.0	17.0
Other intangible assets, finite-lived, net carrying value	105.4
Total other intangible assets	320.1	218.8
Total other intangible assets	281.1	201.8	215.9
Total goodwill and other intangible assets	759.3	387.6
Goodwill and other intangible assets	720.3	370.6	$ 392.4
Other Operations
Goodwill [Line Items]
Goodwill	44.4	44.4
Other intangible assets, finite-lived, acquisition date fair value	41.2	40.9
Other intangible assets, accumulated amortization	20.8	15.5
Other intangible assets, finite-lived, net carrying value	20.4	25.4
Goodwill and other intangible assets	64.8	69.8
Ark | Operating Segments
Goodwill [Line Items]
Goodwill and other intangible assets	292.5	292.5
Ark | Operating Segments | Ark Insurance Holdings Limited - Segment
Goodwill [Line Items]
Goodwill	116.8	116.8
Ark | Operating Segments | Ark Insurance Holdings Limited - Segment | Underwriting Capacity
Goodwill [Line Items]
Indefinite-lived intangible assets	175.7	175.7
Kudu | Operating Segments
Goodwill [Line Items]
Goodwill	7.6	7.6
Goodwill and other intangible assets	8.0	8.3
Bamboo | Operating Segments
Goodwill [Line Items]
Goodwill	270.4	0.0
Other intangible assets, finite-lived, acquisition date fair value	101.0	0.0
Other intangible assets, accumulated amortization	16.4	0.0
Other intangible assets, finite-lived, net carrying value	84.6	0.0
Goodwill and other intangible assets	$ 355.0	0.0
Trade names | Other Operations
Goodwill [Line Items]
Acquired 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, accumulated amortization	5.6	4.1
Other intangible assets, finite-lived, net carrying value	$ 7.7	9.2
Trade names | Kudu | Operating Segments
Goodwill [Line Items]
Acquired 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, accumulated amortization	1.8	1.5
Other intangible assets, finite-lived, net carrying value	$ 0.4	0.7
Trade names | Bamboo | Operating Segments
Goodwill [Line Items]
Acquired 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, accumulated amortization	2.3	0.0
Other intangible assets, finite-lived, net carrying value	$ 21.2	0.0
Agency relationships | Bamboo | Operating Segments
Goodwill [Line Items]
Acquired 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, accumulated amortization	12.1	0.0
Other intangible assets, finite-lived, net carrying value	$ 60.3	0.0
Developed technology | Bamboo | Operating Segments
Goodwill [Line Items]
Acquired 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, accumulated amortization	1.6	0.0
Other intangible assets, finite-lived, net carrying value	$ 3.1	0.0
Customer relationships | Other Operations
Goodwill [Line Items]
Acquired 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, accumulated amortization	13.6	10.3
Other intangible assets, finite-lived, net carrying value	11.2	14.5
Total other intangible assets
Goodwill [Line Items]
Other intangible assets, accumulated amortization	39.0	17.0
Total other intangible assets	$ 281.1	201.8
Total other intangible assets | Other Operations
Goodwill [Line Items]
Acquired 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, accumulated amortization	1.6	1.1
Other intangible assets, finite-lived, net carrying value	$ 1.5	1.7
Total other intangible assets | Bamboo | Operating Segments
Goodwill [Line Items]
Acquired 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, accumulated amortization	0.4	0.0
Other intangible assets, finite-lived, net carrying value	$ 0.0	$ 0.0