Investments - Net Realized Gains (Losses), Including Impairment, Recognized in the Consolidated Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Net realized (losses) gains on investments related to sales and other and fair value changes to equity securities:
Total net realized losses on investments related to sales and other and fair value changes to equity securities	$ (12.6)	$ (21.7)	$ (28.2)	$ (34.7)
Net realized losses related to impairments:
Total net realized losses related to impairments	2.4	(0.0)	(3.2)	(3.0)
Total net realized losses on investments and fair value changes to equity securities	(10.2)	(21.7)	(31.4)	(37.7)
Equity securities, realized gain	0.4	0.0	0.5	2.5
Equity investment, impairment loss	0.0	0.0	0.0	3.0
Fixed maturity securities
Net realized (losses) gains on investments related to sales and other and fair value changes to equity securities:
Total net realized losses on investments related to sales and other and fair value changes to equity securities	(15.2)	(21.2)	(28.2)	(38.2)
Net realized losses related to impairments:
Total net realized losses related to impairments	2.5	0.0	0.0	0.0
Equity securities
Net realized (losses) gains on investments related to sales and other and fair value changes to equity securities:
Total net realized losses on investments related to sales and other and fair value changes to equity securities	2.7	(1.1)	0.0	1.7
Commercial mortgage loans on real estate
Net realized (losses) gains on investments related to sales and other and fair value changes to equity securities:
Total net realized losses on investments related to sales and other and fair value changes to equity securities	0.0	0.3	0.3	1.7
Other investments
Net realized (losses) gains on investments related to sales and other and fair value changes to equity securities:
Total net realized losses on investments related to sales and other and fair value changes to equity securities	(0.1)	0.3	(0.3)	0.1
Net realized losses related to impairments:
Total net realized losses related to impairments	$ (0.1)	$ 0.0	$ (3.2)	$ (3.0)

Investment Impairment Losses included in Net Realized Gains (Losses)

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Amount of impairment loss on investment in equity security without readily determinable fair value.

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Amount of gain from upward price adjustment on investment in equity security without readily determinable fair value.

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Amount of realized and unrealized gain (loss) on investment.

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