Loss and loss adjustment expense reserves - Summary of loss and loss adjustment expense reserves (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Insurance [Abstract]
Case loss and loss adjustment expense reserves	$ 1,916.8	$ 265.6
Incurred but not reported loss and loss adjustment expense reserves	2,868.2	1,043.5
Unallocated loss adjustment expense reserves	55.8	0.0
Deferred gains on retroactive reinsurance contracts	0.6	1.0
Loss and loss adjustment expense reserves	$ 4,841.4	$ 1,310.1	$ 1,111.7	$ 937.2