Third party reinsurance - Gross and net recoverable amounts by the reinsurer's credit rating and the percentage of total recoverables (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	$ 1,215.3	$ 14.4	$ 5.5	$ 2.0
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	$ 1,215.3	$ 14.4
% of total	100.00%	100.00%
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | Collateral
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	$ 560.2	$ 7.2
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | Net
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	655.1	7.2
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | AAA
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net		$ 0.2
% of total		2.80%
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | AAA | Collateral
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net		$ 0.0
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | AAA | Net
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net		0.2
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | AA
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	$ 149.5	$ 0.3
% of total	22.60%	4.20%
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | AA | Collateral
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	$ 1.7	$ 0.0
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | AA | Net
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	147.8	0.3
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | A
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	$ 360.8	$ 2.5
% of total	52.60%	34.70%
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | A | Collateral
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	$ 16.0	$ 0.0
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | A | Net
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	344.8	2.5
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | BBB or lower
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	$ 212.3
% of total	12.50%
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | BBB or lower | Collateral
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	$ 130.5
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | BBB or lower | Net
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	81.8
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | Not rated
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	$ 492.7	$ 11.4
% of total	12.30%	58.30%
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | Not rated | Pallas Reinsurance Company Ltd.
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	$ 355.9
% of total	29.30%
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | Not rated | Collateral
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	$ 412.0	$ 7.2
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | Not rated | Collateral | Pallas Reinsurance Company Ltd.
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	$ 355.9
% of total	100.00%
Loss and loss adjustment expenses recoverable | Reinsurer Concentration Risk | Not rated | Net
Ceded Credit Risk [Line Items]
Loss and loss adjustment expenses recoverable, net	$ 80.7	$ 4.2