Investments - OTTI losses Recognized in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
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)	$ (0.4)
Debt Securities, Available-for-sale, Allowance for Credit Loss	7.5	3.3	$ 0.0
Foreign Governments
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	(0.3)	(0.2)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.3	0.0
Corporate Bonds and Notes
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	(0.8)	(0.2)
Debt Securities, Available-for-sale, Allowance for Credit Loss	$ 7.5	$ 3.0	$ 0.0