Reserves for unpaid losses and loss adjustment expenses Historical Claims Duration (Details)	Dec. 31, 2018 Rate
Property [Member]
Historical Claims Duration [Line Items]
Historical Claims Duration, Year One	60.20%
Historical Claims Duration, Year Two	38.30%
Historical Claims Duration, Year Three	4.90%
Historical Claims Duration, Year Four	1.90%
Historical Claims Duration, Year Five	0.00%
Casualty - claims made [Member]
Historical Claims Duration [Line Items]
Historical Claims Duration, Year One	4.00%
Historical Claims Duration, Year Two	19.60%
Historical Claims Duration, Year Three	25.50%
Historical Claims Duration, Year Four	11.00%
Historical Claims Duration, Year Five	21.30%
Historical Claims Duration, Year Six	8.90%
Historical Claims Duration, Year Seven	0.90%
Historical Claims Duration, Year Eight	0.40%
Historical Claims Duration, Year Nine	0.00%
Casualty - occurrence [Member]
Historical Claims Duration [Line Items]
Historical Claims Duration, Year One	2.30%
Historical Claims Duration, Year Two	10.20%
Historical Claims Duration, Year Three	13.50%
Historical Claims Duration, Year Four	19.50%
Historical Claims Duration, Year Five	19.00%
Historical Claims Duration, Year Six	11.20%
Historical Claims Duration, Year Seven	7.20%
Historical Claims Duration, Year Eight	14.10%
Historical Claims Duration, Year Nine	8.30%