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Investments - Fair Value and Gross Unrealized Losses (Details) $ in Millions	Sep. 30, 2023 USD ($)	Sep. 30, 2023 security	Sep. 30, 2023	Sep. 30, 2023 position	Dec. 31, 2022 USD ($)	Dec. 31, 2022 security	Dec. 31, 2022	Dec. 31, 2022 position
Fixed maturity securities, Gross Unrealized Losses
Number of positions with a gross unrealized loss, 12 Months or Less (security) | security		669				2,515
Number of positions with a gross unrealized loss, more than 12 months (security)		2,474		2,474		587		587
Number of positions with a gross unrealized loss, Total (security) | security		3,143				3,102
Fair value as a percentage of total fixed maturities and equity securities fair value, 12 Months or Less (percent)			18.20%				68.20%
Fair value as a percentage of total fixed maturities and equity securities fair value, more than 12 months (percent)			68.20%				14.20%
Fair value as a percentage of total fixed maturities and equity securities fair value, Total (percent)			86.40%				82.40%
Total
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	$ 901.5				$ 3,533.7
Fixed maturity securities, Fair Value, More than 12 Months	3,384.9				734.2
Fixed maturity securities, Fair Value, Total	4,286.4				4,267.9
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	44.0				461.8
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	658.8				145.1
Fixed maturity securities, Gross Unrealized Losses, Total	702.8				606.9
Mortgage-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	137.1				458.3
Fixed maturity securities, Fair Value, More than 12 Months	438.2				52.6
Fixed maturity securities, Fair Value, Total	575.3				510.9
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	5.3				54.4
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	91.9				14.7
Fixed maturity securities, Gross Unrealized Losses, Total	97.2				69.1
Other
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	91.9				242.7
Fixed maturity securities, Fair Value, More than 12 Months	268.5				65.8
Fixed maturity securities, Fair Value, Total	360.4				308.5
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	3.7				34.1
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	81.3				33.7
Fixed maturity securities, Gross Unrealized Losses, Total	85.0				67.8
Municipal bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	331.1				911.6
Fixed maturity securities, Fair Value, More than 12 Months	729.9				42.2
Fixed maturity securities, Fair Value, Total	1,061.0				953.8
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	13.7				113.7
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	146.1				14.4
Fixed maturity securities, Gross Unrealized Losses, Total	159.8				128.1
Foreign government bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	1.4				32.7
Fixed maturity securities, Fair Value, More than 12 Months	23.1				0.4
Fixed maturity securities, Fair Value, Total	24.5				33.1
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	0.1				1.4
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	1.5				0.2
Fixed maturity securities, Gross Unrealized Losses, Total	1.6				1.6
Corporate bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	204.9				1,345.0
Fixed maturity securities, Fair Value, More than 12 Months	1,120.8				148.9
Fixed maturity securities, Fair Value, Total	1,325.7				1,493.9
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	18.6				221.1
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	286.0				51.1
Fixed maturity securities, Gross Unrealized Losses, Total	304.6				272.2
Other asset-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	135.1				543.4
Fixed maturity securities, Fair Value, More than 12 Months	804.4				424.3
Fixed maturity securities, Fair Value, Total	939.5				967.7
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	2.6				37.1
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	52.0				31.0
Fixed maturity securities, Gross Unrealized Losses, Total	$ 54.6				$ 68.1

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