Loss and loss adjustment expense reserves - Reconciliation of loss development information to loss and loss adjustment expense reserves (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net reserves for loss and allocated loss adjustment expenses, end of year	$ 3,503.6
Total loss and allocated loss adjustment expenses recoverable	1,215.3	$ 14.4	$ 5.5	$ 2.0
Unallocated loss adjustment expense reserves	55.8
Other items, net	65.3
Deferred charges on retroactive reinsurance contracts	1.4	6.0	6.7	3.8
Gross reserves for loss and loss adjustment expenses, end of year	4,841.4	$ 1,310.1	$ 1,111.7	$ 937.2
Corporate
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net reserves for loss and allocated loss adjustment expenses, end of year	402.1
Total loss and allocated loss adjustment expenses recoverable	468.9
Reinsurance | Property
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total loss and allocated loss adjustment expenses recoverable	1,215.3
Reinsurance | Operating Segments | Aviation & Space
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net reserves for loss and allocated loss adjustment expenses, end of year	132.8
Total loss and allocated loss adjustment expenses recoverable	42.1
Reinsurance | Operating Segments | Casualty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net reserves for loss and allocated loss adjustment expenses, end of year	1,214.5
Total loss and allocated loss adjustment expenses recoverable	10.1
Reinsurance | Operating Segments | Contingency
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net reserves for loss and allocated loss adjustment expenses, end of year	18.4
Reinsurance | Operating Segments | Credit & Bond
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net reserves for loss and allocated loss adjustment expenses, end of year	76.9
Total loss and allocated loss adjustment expenses recoverable	9.9
Reinsurance | Operating Segments | Marine & Energy
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net reserves for loss and allocated loss adjustment expenses, end of year	64.8
Total loss and allocated loss adjustment expenses recoverable	8.3
Reinsurance | Operating Segments | Mortgage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net reserves for loss and allocated loss adjustment expenses, end of year	32.4
Total loss and allocated loss adjustment expenses recoverable	2.8
Reinsurance | Operating Segments | Property
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net reserves for loss and allocated loss adjustment expenses, end of year	1,195.7
Total loss and allocated loss adjustment expenses recoverable	547.3
Insurance & Services | Operating Segments | A&H
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net reserves for loss and allocated loss adjustment expenses, end of year	176.8
Total loss and allocated loss adjustment expenses recoverable	64.4
Insurance & Services | Operating Segments | Environmental
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net reserves for loss and allocated loss adjustment expenses, end of year	8.5
Total loss and allocated loss adjustment expenses recoverable	6.8
Insurance & Services | Operating Segments | Workers’ Compensation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net reserves for loss and allocated loss adjustment expenses, end of year	118.2
Total loss and allocated loss adjustment expenses recoverable	10.2
Insurance & Services | Operating Segments | Other
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net reserves for loss and allocated loss adjustment expenses, end of year	62.5
Total loss and allocated loss adjustment expenses recoverable	$ 44.5