Investment Securities - Debt Securities Issued by Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	$ 1,073.1	$ 1,045.0
AAA
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	11.4	11.5
AA
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	83.5	83.8
A
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	562.4	552.4
BBB
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	407.5	390.9
BB
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	1.8	0.0
Other
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	$ 6.5	$ 6.4