Reserves for unpaid losses and loss adjustment expenses - By segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Reserves for unpaid losses and loss adjustment expenses
Prior years losses	$ 4.2	$ 16.6
Natural catastrophes
Reserves for unpaid losses and loss adjustment expenses
Prior years losses		15.6
Global Reinsurance
Reserves for unpaid losses and loss adjustment expenses
Prior years losses	9.1	10.3
Global A&H
Reserves for unpaid losses and loss adjustment expenses
Prior years losses	$ (4.2)	5.1
Runoff & Other
Reserves for unpaid losses and loss adjustment expenses
Prior years losses		$ 1.2