Income taxes - Deferred tax assets and liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Non-U.S. net operating loss carryforwards	$ 273.3	$ 0.0	$ 0.0
Tax credit carryforwards	26.2	0.0	0.0
U.S. federal net operating loss and capital carryforwards	19.4	10.1	9.3
Purchase accounting	17.8	0.0	0.0
Unearned premiums	13.4	5.5	1.8
Discounting of loss and loss adjustment expense reserves	7.6	1.0	0.8
Investment basis differences	5.9	0.0	0.0
Deferred interest	3.9	0.0	0.0
Incentive compensation and benefit accruals	5.7	2.6	1.2
Allowance for doubtful accounts	2.9	0.0	0.0
Other items	5.2	0.0	0.0
Total gross deferred tax assets	381.3	19.2	13.1
Valuation allowance	(113.3)	0.0	0.0
Total adjusted deferred tax asset	268.0	19.2	13.1
Deferred tax liabilities:
Safety reserve	150.1	0.0	0.0
Intangible assets	14.3	0.0	0.0
Deferred acquisition costs	6.7	8.3	1.4
Unrealized gains on investments	3.8	10.5	3.4
Foreign currency translation on investments	1.7	0.0	0.0
Other Items	4.8	0.0	0.0
Total deferred tax liabilities	181.4	18.8	4.8
Net deferred tax assets	$ 86.6	$ 0.4	$ 8.3