Third party reinsurance - Breakdown of written and earned premiums and loss and loss adjustment expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Written premiums:
Direct	$ 1,403.9	$ 718.0	$ 19.0
Assumed	2,005.8	1,518.5	569.5
Gross premiums written	3,409.7	2,236.5	588.5
Ceded	(860.5)	(502.3)	(46.3)
Net premiums written	2,549.2	1,734.2	542.2
Premiums earned:
Direct	1,153.6	600.8	1.0
Assumed	1,915.2	1,598.5	638.8
Gross premiums earned	3,068.8	2,199.3	639.8
Ceded	(750.7)	(482.3)	(29.0)
Net premiums earned	2,318.1	1,717.0	610.8
Loss and loss adjustment expense:
Direct	778.0	349.3	0.8
Assumed	1,386.8	1,506.1	483.2
Loss and loss adjustment expense incurred	2,164.8	1,855.4	484.0
Ceded	(576.4)	(528.9)	(18.7)
Loss and loss adjustment expense incurred, net	$ 1,588.4	$ 1,326.5	$ 465.3