Third party reinsurance - Highest gross recoverable amounts by reinsurer (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Third party reinsurance
Gross recoverable balance	$ 405.2	$ 337.2
AA
Third party reinsurance
Gross recoverable balance	115.1	116.9
A
Third party reinsurance
Gross recoverable balance	212.9	153.9
Not rated
Third party reinsurance
Gross recoverable balance	57.7	53.0
General Insurance Corporation of India (1) | A-
Third party reinsurance
Gross recoverable balance	$ 82.7	$ 17.3
Percentage of Total	20.00%	5.00%
Percentage Collateralized	97.00%	11.00%
Swiss Reinsurance Company Ltd | AA-
Third party reinsurance
Gross recoverable balance	$ 44.1	$ 45.5
Percentage of Total	11.00%	13.00%
Percentage Collateralized	0.00%	0.00%
Berkshire Hathaway, Inc | AA+
Third party reinsurance
Gross recoverable balance	$ 39.3	$ 41.1
Percentage of Total	10.00%	12.00%
Percentage Collateralized	1.00%	0.00%
Argo Capital Group Ltd | A-
Third party reinsurance
Gross recoverable balance	$ 16.7	$ 18.2
Percentage of Total	4.00%	5.00%
Percentage Collateralized	87.00%	78.00%
Lloyd's of London | A+
Third party reinsurance
Gross recoverable balance	$ 13.4	$ 19.4
Percentage of Total	3.00%	6.00%
Percentage Collateralized	5.00%	16.00%