Loss and loss adjustment expense reserves - Activity in the loss and loss adjustment expense reserves (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
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves for loss and loss adjustment expenses, beginning of year				$ 1,310.1	$ 1,310.1	$ 1,310.1	$ 1,310.1	$ 1,111.7	$ 937.2
Less: loss and loss adjustment expenses recoverable, beginning of year				(14.4)	(14.4)	(14.4)	(14.4)	(5.5)	(2.0)
Less: deferred charges on retroactive reinsurance contracts				(6.0)	(6.0)	(6.0)	(6.0)	(6.7)	(3.8)
Net reserves for loss and loss adjustment expenses, beginning of year				1,289.7	1,289.7	1,289.7	1,289.7	1,099.5	931.4
Increase (decrease) in net loss and loss adjustment expenses incurred in respect of losses occurring in:
Current year							1,369.1	431.5	490.0
Prior years							(42.6)	33.8	(86.5)
Total incurred loss and loss adjustment expenses	$ 351.4	$ 577.3	$ 250.9	$ 146.9	$ 397.8	$ 975.1	1,326.5	465.3	403.5
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Current year							(271.2)	(73.6)	(63.6)
Prior years							(1,178.9)	(209.5)	(188.4)
Total net paid losses							(1,450.1)	(283.1)	(252.0)
Foreign currency translation							(9.2)	8.0	16.6
Amounts acquired as a result of Sirius Group acquisition							2,467.8	0.0	0.0
Net reserves for loss and loss adjustment expenses, end of year	3,624.7						3,624.7	1,289.7	1,099.5
Plus: loss and loss adjustment expenses recoverable, end of year	1,215.3						1,215.3	14.4	5.5
Plus: deferred charges on retroactive reinsurance contracts	1.4						1.4	6.0	6.7
Gross reserves for loss and loss adjustment expenses, end of year	$ 4,841.4						4,841.4	$ 1,310.1	$ 1,111.7
Amortization associated with the fair value adjustment							$ 15.0