Reserves for unpaid losses and loss adjustment expenses Historical Claims Duration (Details)	Dec. 31, 2019 Rate
Property [Member]
Historical Claims Duration [Line Items]
Historical Claims Duration, Year One	70.30%
Historical Claims Duration, Year Two	22.10%
Historical Claims Duration, Year Three	5.90%
Historical Claims Duration, Year Four	1.90%
Historical Claims Duration, Year Five	0.40%
Casualty - claims made [Member]
Historical Claims Duration [Line Items]
Historical Claims Duration, Year One	4.50%
Historical Claims Duration, Year Two	20.20%
Historical Claims Duration, Year Three	25.30%
Historical Claims Duration, Year Four	12.50%
Historical Claims Duration, Year Five	20.40%
Historical Claims Duration, Year Six	9.50%
Historical Claims Duration, Year Seven	0.90%
Historical Claims Duration, Year Eight	0.20%
Historical Claims Duration, Year Nine	0.60%
Historical Claims Duration, Year Ten	0.00%
Casualty - occurrence [Member]
Historical Claims Duration [Line Items]
Historical Claims Duration, Year One	2.40%
Historical Claims Duration, Year Two	9.90%
Historical Claims Duration, Year Three	13.70%
Historical Claims Duration, Year Four	19.00%
Historical Claims Duration, Year Five	18.30%
Historical Claims Duration, Year Six	12.10%
Historical Claims Duration, Year Seven	6.30%
Historical Claims Duration, Year Eight	11.10%
Historical Claims Duration, Year Nine	5.10%
Historical Claims Duration, Year Ten	0.00%