Investment Securities - Debt securities issued by corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 974.2	$ 992.9
AAA
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	11.6	12.0
AA
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	87.2	85.0
A
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	502.3	490.4
BBB
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	366.4	396.8
Other
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 6.7	$ 8.7