Derivatives - Listing currency, fair value and notional amounts of underwriting-related derivatives (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Derivative [Line Items]
Derivative liabilities, Fair value	$ 3.2	$ 1.0
Reinsurance contracts accounted for as derivative assets | GBP
Derivative [Line Items]
Derivative assets, Fair value	1.2	1.2
Derivative assets, Notional Amounts	49.3	4.2
Derivative liabilities, Fair value	0.1	1.0
Derivative liabilities, Notional Amounts	37.4	15.7
Underwriting-related derivatives
Derivative [Line Items]
Derivative assets, Fair value	1.2	1.2
Derivative assets, Notional Amounts	49.3	4.2
Derivative liabilities, Fair value	0.1	1.0
Derivative liabilities, Notional Amounts	$ 37.4	$ 15.7