Reserves for unpaid losses and loss adjustment expenses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Summary of the loss and LAE reserve activities
Gross beginning balance	$ 2,519.6	$ 1,976.3	$ 2,331.5	$ 2,016.7
Less beginning reinsurance recoverable on unpaid losses	(444.2)	(349.3)	(410.3)	(350.2)
Net loss and LAE reserve balance	2,075.4	1,627.0	1,921.2	1,666.5
Losses and LAE incurred relating to:
Current year losses	239.1	214.1	662.0	381.4
Prior years losses	1.2	63.9	5.4	80.5
Total net incurred losses and LAE	240.3	278.0	667.4	461.9
Foreign currency translation adjustment to net loss and LAE reserves	13.2	3.4	(8.6)	0.0
Accretion of fair value adjustment to net loss and LAE reserves	0.0	0.1	0.0	0.1
Loss and LAE paid relating to:
Current year losses	24.5	120.9	67.6	157.1
Prior years losses	231.5	121.7	439.5	305.5
Total loss and LAE payments	256.0	242.6	507.1	462.6
Net ending balance	2,072.9	1,665.9	2,072.9	1,665.9
Plus ending reinsurance recoverable on unpaid losses	442.2	357.4	442.2	357.4
Gross ending balance	$ 2,515.1	$ 2,023.3	$ 2,515.1	$ 2,023.3