Reserves for unpaid losses and loss adjustment expenses - By segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Reserves for unpaid losses and loss adjustment expenses
Prior years losses	$ 1.2	$ 63.9	$ 5.4	$ 80.5
Natural catastrophes
Reserves for unpaid losses and loss adjustment expenses
Prior years losses	7.6	46.4		46.4
Global Reinsurance
Reserves for unpaid losses and loss adjustment expenses
Prior years losses	4.5	61.6	13.6	71.8
Global A&H
Reserves for unpaid losses and loss adjustment expenses
Prior years losses	3.3		7.5	4.5
Casualty Reinsurance
Reserves for unpaid losses and loss adjustment expenses
Prior years losses	4.8		5.6
Runoff & Other
Reserves for unpaid losses and loss adjustment expenses
Prior years losses		$ 3.0		4.2
Aviation and Space
Reserves for unpaid losses and loss adjustment expenses
Prior years losses			$ 8.8
Typhoon Jebi
Reserves for unpaid losses and loss adjustment expenses
Prior years losses				7.3
Hurricane Florence
Reserves for unpaid losses and loss adjustment expenses
Prior years losses				7.2
Hurricane Irma
Reserves for unpaid losses and loss adjustment expenses
Prior years losses				$ 6.3
Other property
Reserves for unpaid losses and loss adjustment expenses
Prior years losses	$ 8.2