Investments (Category and Duration of Gross Unrealized Losses on Fixed Maturity Securities and Equity Securities) (Details) - Fixed Maturities - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, Less than 12 months, Fair Value	$ 2,003.9	$ 243.0
Fixed maturity securities, less than 12 months, unrealized losses	(33.1)	(4.4)
Fixed maturity securities, 12 months or more, Fair value	47.7	56.0
Fixed maturity securities, 12 months or more, unrealized losses	(2.1)	(2.9)
Fixed maturity securities, Total, Fair Value	2,051.6	299.0
Fixed maturity securities, Unrealized Losses	(35.2)	(7.3)
U.S. government and government agencies and authorities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, Less than 12 months, Fair Value	31.5
Fixed maturity securities, less than 12 months, unrealized losses	(0.1)
Fixed maturity securities, 12 months or more, Fair value	0.0
Fixed maturity securities, 12 months or more, unrealized losses	0.0
Fixed maturity securities, Total, Fair Value	31.5
Fixed maturity securities, Unrealized Losses	(0.1)
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, Less than 12 months, Fair Value	48.1	6.1
Fixed maturity securities, less than 12 months, unrealized losses	(0.7)	(0.1)
Fixed maturity securities, 12 months or more, Fair value	0.0	0.0
Fixed maturity securities, 12 months or more, unrealized losses	0.0	0.0
Fixed maturity securities, Total, Fair Value	48.1	6.1
Fixed maturity securities, Unrealized Losses	(0.7)	(0.1)
Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, Less than 12 months, Fair Value	216.0	28.3
Fixed maturity securities, less than 12 months, unrealized losses	(4.1)	(0.1)
Fixed maturity securities, 12 months or more, Fair value	4.0	0.0
Fixed maturity securities, 12 months or more, unrealized losses	(0.1)	0.0
Fixed maturity securities, Total, Fair Value	220.0	28.3
Fixed maturity securities, Unrealized Losses	(4.2)	(0.1)
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, Less than 12 months, Fair Value	257.7	54.5
Fixed maturity securities, less than 12 months, unrealized losses	(2.1)	(0.2)
Fixed maturity securities, 12 months or more, Fair value	9.8	37.4
Fixed maturity securities, 12 months or more, unrealized losses	(0.2)	(0.6)
Fixed maturity securities, Total, Fair Value	267.5	91.9
Fixed maturity securities, Unrealized Losses	(2.3)	(0.8)
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, Less than 12 months, Fair Value	274.8	28.2
Fixed maturity securities, less than 12 months, unrealized losses	(2.9)	(0.7)
Fixed maturity securities, 12 months or more, Fair value	2.0	3.3
Fixed maturity securities, 12 months or more, unrealized losses	(0.4)	(0.7)
Fixed maturity securities, Total, Fair Value	276.8	31.5
Fixed maturity securities, Unrealized Losses	(3.3)	(1.4)
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, Less than 12 months, Fair Value	94.0	23.9
Fixed maturity securities, less than 12 months, unrealized losses	(1.5)	(0.1)
Fixed maturity securities, 12 months or more, Fair value	10.0	1.5
Fixed maturity securities, 12 months or more, unrealized losses	(0.2)	(0.1)
Fixed maturity securities, Total, Fair Value	104.0	25.4
Fixed maturity securities, Unrealized Losses	(1.7)	(0.2)
U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, Less than 12 months, Fair Value	687.8	71.9
Fixed maturity securities, less than 12 months, unrealized losses	(13.1)	(2.9)
Fixed maturity securities, 12 months or more, Fair value	15.2	13.8
Fixed maturity securities, 12 months or more, unrealized losses	(0.9)	(1.5)
Fixed maturity securities, Total, Fair Value	703.0	85.7
Fixed maturity securities, Unrealized Losses	(14.0)	(4.4)
Foreign corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, Less than 12 months, Fair Value	394.0	30.1
Fixed maturity securities, less than 12 months, unrealized losses	(8.6)	(0.3)
Fixed maturity securities, 12 months or more, Fair value	6.7	0.0
Fixed maturity securities, 12 months or more, unrealized losses	(0.3)	0.0
Fixed maturity securities, Total, Fair Value	400.7	30.1
Fixed maturity securities, Unrealized Losses	$ (8.9)	$ (0.3)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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