Reserves for unpaid losses and loss adjustment expenses - By segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reserves for unpaid losses and loss adjustment expenses
Prior years losses	$ (0.1)	$ 6.4	$ 5.3	$ 86.9
Natural catastrophes
Reserves for unpaid losses and loss adjustment expenses
Prior years losses			22.5
Other property
Reserves for unpaid losses and loss adjustment expenses
Prior years losses			21.6
U S Specialty
Reserves for unpaid losses and loss adjustment expenses
Prior years losses	0.8		(2.0)
Runoff & Other
Reserves for unpaid losses and loss adjustment expenses
Prior years losses	0.4			5.4
Global Reinsurance
Reserves for unpaid losses and loss adjustment expenses
Prior years losses	$ 1.3	10.9	14.9	$ 82.7
Global A&H
Reserves for unpaid losses and loss adjustment expenses
Prior years losses		$ 5.8	(7.6)
Casualty Reinsurance
Reserves for unpaid losses and loss adjustment expenses
Prior years losses			9.2
Aviation and Space
Reserves for unpaid losses and loss adjustment expenses
Prior years losses			$ 10.1