Reserves for unpaid losses and loss adjustment expenses - Reconciliation of liabilities for unpaid loss and LAE (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reserves for unpaid losses and loss adjustment expenses
Total unpaid loss and LAE reserves, net of reinsurance	$ 1,631.0
Unallocated LAE	35.5
Liability for Unpaid Claims and Claims Adjustment Expense, Net, Total	1,666.5	$ 1,578.8	$ 1,328.6	$ 1,361.3
Reinsurance recoverable on unpaid losses
Total reinsurance recoverable on unpaid losses	350.2	319.7	291.5	283.1
Total unpaid loss and LAE reserves	2,016.7	$ 1,898.5	$ 1,620.1	$ 1,644.4
Other property
Reserves for unpaid losses and loss adjustment expenses
Total unpaid loss and LAE reserves, net of reinsurance	450.8
Reinsurance recoverable on unpaid losses
Total reinsurance recoverable on unpaid losses	104.6
Property Catastrophe Excess
Reserves for unpaid losses and loss adjustment expenses
Total unpaid loss and LAE reserves, net of reinsurance	245.7
Reinsurance recoverable on unpaid losses
Total reinsurance recoverable on unpaid losses	53.4
Agriculture
Reserves for unpaid losses and loss adjustment expenses
Total unpaid loss and LAE reserves, net of reinsurance	40.9
Reinsurance recoverable on unpaid losses
Total reinsurance recoverable on unpaid losses	0.5
Global Accident & Health
Reserves for unpaid losses and loss adjustment expenses
Total unpaid loss and LAE reserves, net of reinsurance	148.2
Reinsurance recoverable on unpaid losses
Total reinsurance recoverable on unpaid losses	45.0
Aviation & Space
Reserves for unpaid losses and loss adjustment expenses
Total unpaid loss and LAE reserves, net of reinsurance	86.6
Reinsurance recoverable on unpaid losses
Total reinsurance recoverable on unpaid losses	18.6
Trade credit
Reserves for unpaid losses and loss adjustment expenses
Total unpaid loss and LAE reserves, net of reinsurance	42.1
Reinsurance recoverable on unpaid losses
Total reinsurance recoverable on unpaid losses	10.9
Environmental
Reserves for unpaid losses and loss adjustment expenses
Total unpaid loss and LAE reserves, net of reinsurance	0.4
Reinsurance recoverable on unpaid losses
Total reinsurance recoverable on unpaid losses	0.4
Marine
Reserves for unpaid losses and loss adjustment expenses
Total unpaid loss and LAE reserves, net of reinsurance	58.9
Reinsurance recoverable on unpaid losses
Total reinsurance recoverable on unpaid losses	8.9
Surety
Reserves for unpaid losses and loss adjustment expenses
Total unpaid loss and LAE reserves, net of reinsurance	4.5
Reinsurance recoverable on unpaid losses
Total reinsurance recoverable on unpaid losses	0.5
Contingency
Reserves for unpaid losses and loss adjustment expenses
Total unpaid loss and LAE reserves, net of reinsurance	13.7
Reinsurance recoverable on unpaid losses
Total reinsurance recoverable on unpaid losses	4.9
Casualty
Reserves for unpaid losses and loss adjustment expenses
Total unpaid loss and LAE reserves, net of reinsurance	65.4
Runoff & Other
Reserves for unpaid losses and loss adjustment expenses
Total unpaid loss and LAE reserves, net of reinsurance	473.7
Reinsurance recoverable on unpaid losses
Total reinsurance recoverable on unpaid losses	$ 102.5