Investment Securities - Debt securities issued by corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 974.5	$ 992.9
AAA
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	11.2	12.0
AA
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	90.5	85.0
A
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	524.0	490.4
BBB
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	343.0	396.8
Other
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 5.8	$ 8.7