Reserves for unpaid losses and loss adjustment expenses - By segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Reserves for unpaid losses and loss adjustment expenses
Unfavorable (favorable) loss reserve development	$ 16.6	$ (2.5)
Global Property
Reserves for unpaid losses and loss adjustment expenses
Unfavorable (favorable) loss reserve development	11.6	17.2
Global Property | Natural catastrophes
Reserves for unpaid losses and loss adjustment expenses
Unfavorable (favorable) loss reserve development		6.4
Hurricanes Harvey, Irma and Maria
Reserves for unpaid losses and loss adjustment expenses
Unfavorable (favorable) loss reserve development	15.6
Global A&H
Reserves for unpaid losses and loss adjustment expenses
Unfavorable (favorable) loss reserve development	5.1	(3.7)
Runoff & Other
Reserves for unpaid losses and loss adjustment expenses
Unfavorable (favorable) loss reserve development	1.2	(8.9)
Specialty & Casualty
Reserves for unpaid losses and loss adjustment expenses
Unfavorable (favorable) loss reserve development	$ (1.3)	$ (7.1)