Reserves for unpaid losses and loss adjustment expenses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Summary of the loss and LAE reserve activities
Gross beginning balance	$ 2,331.5	$ 2,016.7
Less beginning reinsurance recoverable on unpaid losses	(410.3)	(350.2)
Net loss and LAE reserve balance	1,921.2	1,666.5
Losses and LAE incurred relating to:
Current year losses	422.9	167.3
Prior years losses	4.2	16.6
Total net incurred losses and LAE	427.1	183.9
Foreign currency translation adjustment to net loss and LAE reserves	(21.8)	(3.4)
Loss and LAE paid relating to:
Current year losses	43.1	36.2
Prior years losses	208.0	183.8
Total loss and LAE payments	251.1	220.0
Net ending balance	2,075.4	1,627.0
Plus ending reinsurance recoverable on unpaid losses	444.2	349.3
Gross ending balance	$ 2,519.6	$ 1,976.3