Investments - Ratings and fair value of debt securities (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Debt and Equity Securities, FV-NI [Line Items]
Debt securities	$ 2,085.6	$ 101.3
AAA
Debt and Equity Securities, FV-NI [Line Items]
Debt securities	696.4	53.2
AA
Debt and Equity Securities, FV-NI [Line Items]
Debt securities	884.1	0.0
A
Debt and Equity Securities, FV-NI [Line Items]
Debt securities	278.5	9.1
BBB
Debt and Equity Securities, FV-NI [Line Items]
Debt securities	153.1	37.7
Other
Debt and Equity Securities, FV-NI [Line Items]
Debt securities	$ 73.5	$ 1.3