Investment Securities - Debt securities issued by corporation (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 2,533.4	$ 2,746.0	$ 2,569.4	$ 2,328.7
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	528.7	467.2
AAA | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	11.2	9.5
AA | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	58.2	73.9
A | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	307.7	288.5
BBB | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	150.0	95.3
BB | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 1.6	$ 0.0