Investments - Investment Category and Duration of Gross Unrealized Losses on Fixed Maturity Securities and Equity Securities (Details) - Fixed maturity securities - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	$ 5,119.7	$ 2,003.9
Fixed maturity securities, less than 12 months, unrealized losses	(568.0)	(33.1)
Fixed maturity securities, 12 months or more, fair value	618.6	47.7
Fixed maturity securities, 12 months or more, unrealized losses	(174.8)	(2.1)
Fixed maturity securities, total, fair value	5,738.3	2,051.6
Fixed maturity securities, unrealized losses	(742.8)	(35.2)
U.S. government and government agencies and authorities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	87.8	31.5
Fixed maturity securities, less than 12 months, unrealized losses	(6.6)	(0.1)
Fixed maturity securities, 12 months or more, fair value	4.1	0.0
Fixed maturity securities, 12 months or more, unrealized losses	(0.5)	0.0
Fixed maturity securities, total, fair value	91.9	31.5
Fixed maturity securities, unrealized losses	(7.1)	(0.1)
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	96.4	48.1
Fixed maturity securities, less than 12 months, unrealized losses	(10.9)	(0.7)
Fixed maturity securities, 12 months or more, fair value	21.8	0.0
Fixed maturity securities, 12 months or more, unrealized losses	(5.7)	0.0
Fixed maturity securities, total, fair value	118.2	48.1
Fixed maturity securities, unrealized losses	(16.6)	(0.7)
Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	306.9	216.0
Fixed maturity securities, less than 12 months, unrealized losses	(17.9)	(4.1)
Fixed maturity securities, 12 months or more, fair value	26.8	4.0
Fixed maturity securities, 12 months or more, unrealized losses	(3.1)	(0.1)
Fixed maturity securities, total, fair value	333.7	220.0
Fixed maturity securities, unrealized losses	(21.0)	(4.2)
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	474.3	257.7
Fixed maturity securities, less than 12 months, unrealized losses	(38.5)	(2.1)
Fixed maturity securities, 12 months or more, fair value	76.7	9.8
Fixed maturity securities, 12 months or more, unrealized losses	(8.4)	(0.2)
Fixed maturity securities, total, fair value	551.0	267.5
Fixed maturity securities, unrealized losses	(46.9)	(2.3)
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	337.9	274.8
Fixed maturity securities, less than 12 months, unrealized losses	(35.5)	(2.9)
Fixed maturity securities, 12 months or more, fair value	73.3	2.0
Fixed maturity securities, 12 months or more, unrealized losses	(15.0)	(0.4)
Fixed maturity securities, total, fair value	411.2	276.8
Fixed maturity securities, unrealized losses	(50.5)	(3.3)
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	413.2	94.0
Fixed maturity securities, less than 12 months, unrealized losses	(45.2)	(1.5)
Fixed maturity securities, 12 months or more, fair value	32.3	10.0
Fixed maturity securities, 12 months or more, unrealized losses	(12.5)	(0.2)
Fixed maturity securities, total, fair value	445.5	104.0
Fixed maturity securities, unrealized losses	(57.7)	(1.7)
U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	2,460.4	687.8
Fixed maturity securities, less than 12 months, unrealized losses	(295.9)	(13.1)
Fixed maturity securities, 12 months or more, fair value	225.8	15.2
Fixed maturity securities, 12 months or more, unrealized losses	(78.2)	(0.9)
Fixed maturity securities, total, fair value	2,686.2	703.0
Fixed maturity securities, unrealized losses	(374.1)	(14.0)
Foreign corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	942.8	394.0
Fixed maturity securities, less than 12 months, unrealized losses	(117.5)	(8.6)
Fixed maturity securities, 12 months or more, fair value	157.8	6.7
Fixed maturity securities, 12 months or more, unrealized losses	(51.4)	(0.3)
Fixed maturity securities, total, fair value	1,100.6	400.7
Fixed maturity securities, unrealized losses	$ (168.9)	$ (8.9)

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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