Income taxes - Reconciliation of taxes at the statutory rate to income tax (benefit) on pre-tax income (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
Income Tax Disclosure [Abstract]
Tax (expense) benefit at the 0% Bermuda statutory rate							$ 0.0	$ 0.0	$ 0.0
Differences in taxes resulting from:
Non-Bermuda earnings							17.6	(8.0)	(0.7)
Foreign currency effects							(19.0)	0.0	0.0
Change in valuation allowance							10.5	0.0	0.0
Change in uncertain tax positions							(8.0)	(0.1)	0.0
Non-taxable/deductible income							7.2	0.0	0.0
Tax rate change							4.3	0.0	0.0
Tax on Safety Reserve							(1.0)	0.0	0.0
State taxes expense							(0.9)	0.0	0.0
Provision-to-return true up							(0.5)	0.0	0.0
Other, net							0.5	0.0	0.0
Total income tax (expense) benefit	$ 17.1	$ 13.0	$ (9.6)	$ (9.8)	$ (19.4)	$ (6.4)	$ 10.7	$ (8.1)	$ (0.7)