Investment Securities - Debt securities issued by corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 969.6	$ 992.9
AAA
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	10.8	12.0
AA
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	84.3	85.0
A
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	468.3	490.4
BBB
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	398.5	396.8
Other
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 7.7	$ 8.7