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, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
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	$ (16.9)	$ (74.8)	$ (25.0)	$ (136.7)
Net realized losses related to impairments:
Other investments	(1.0)	0.0	(3.5)	0.0
Net realized losses related to impairments	(3.1)	(1.6)	(5.6)	(2.1)
Total net realized losses on investments and fair value changes to equity securities	(20.0)	(76.4)	(30.6)	(138.8)
Upward adjustment for equity securities under alternative measurement	0.4	9.5	0.4	19.5
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	(7.8)	(22.6)	(11.8)	(41.1)
Net realized losses related to impairments:
Net realized losses related to impairments	(2.1)	(1.6)	(2.1)	(1.6)
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	(9.8)	(54.0)	(12.9)	(97.2)
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	0.2	0.0	(1.3)	0.2
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	0.5	1.8	1.0	1.4
Net realized losses related to impairments:
Other investments	$ (1.0)	$ 0.0	$ (3.5)	$ (0.5)