Investments - OTTI losses Recognized in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized	$ (0.9)	$ 5.8
Allowance for credit loss, available for sale	8.2	9.6	$ 7.5
Debt Security, Government, Non-US [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized		0.0
Allowance for credit loss, available for sale	0.0	0.0
Corporate Debt Securities [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(1.1)	5.8
Allowance for credit loss, available for sale	$ 7.7	$ 8.9	$ 7.5