Income taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 1.6	$ 1.5
Changes in prior year tax positions		0.1
Acquisition of Sirius Group	0.9
Changes in prior year tax positions	(0.1)
Tax positions taken during the current year	8.3
Ending balance	10.7	1.6
Permanent differences
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	1.1	1.0
Changes in prior year tax positions		0.1
Acquisition of Sirius Group	0.7
Changes in prior year tax positions	(0.1)
Tax positions taken during the current year	8.0
Ending balance	9.7	1.1
Temporary differences
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	0.0	0.0
Changes in prior year tax positions		0.0
Acquisition of Sirius Group	0.1
Changes in prior year tax positions	(0.1)
Tax positions taken during the current year	0.3
Ending balance	0.3	0.0
Interest and penalties
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	0.5	0.5
Changes in prior year tax positions	0.1	0.0
Acquisition of Sirius Group	0.1
Tax positions taken during the current year	0.0
Ending balance	$ 0.7	$ 0.5