Third party reinsurance - Breakdown of written and earned premiums and loss and loss adjustment expense (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2021	Sep. 30, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Written premiums:
Direct							$ 718.0	$ 19.0	$ 0.0
Assumed							1,518.5	569.5	668.4
Gross premiums written							2,236.5	588.5	668.4
Ceded							(502.3)	(46.3)	(11.4)
Net premiums written							1,734.2	542.2	657.0
Premiums earned:
Direct							600.8	1.0	0.0
Assumed							1,598.5	638.8	710.0
Gross premiums earned							2,199.3	639.8	710.0
Ceded							(482.3)	(29.0)	(9.9)
Net premiums earned	$ 519.9	$ 499.6	$ 452.3	$ 245.2	$ 697.5	$ 1,197.1	1,717.0	610.8	700.1
Loss and loss adjustment expense:
Direct							349.3	0.8	0.0
Assumed							1,506.1	483.2	407.5
Loss and loss adjustment expense incurred							1,855.4	484.0	407.5
Ceded							(528.9)	(18.7)	(4.0)
Loss and loss adjustment expense incurred, net							$ 1,326.5	$ 465.3	$ 403.5