Reserves for unpaid losses and loss adjustment expenses Historical Claims Duration (Details)	Dec. 31, 2017 Rate
Property [Member]
Historical Claims Duration [Line Items]
Historical Claims Duration, Year One	67.90%
Historical Claims Duration, Year Two	29.60%
Historical Claims Duration, Year Three	4.90%
Historical Claims Duration, Year Four	0.00%
Historical Claims Duration, Year Five	0.00%
Casualty - claims made [Member]
Historical Claims Duration [Line Items]
Historical Claims Duration, Year One	4.10%
Historical Claims Duration, Year Two	17.70%
Historical Claims Duration, Year Three	24.10%
Historical Claims Duration, Year Four	10.40%
Historical Claims Duration, Year Five	21.90%
Historical Claims Duration, Year Six	10.50%
Historical Claims Duration, Year Seven	0.00%
Historical Claims Duration, Year Eight	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	12.80%
Historical Claims Duration, Year Four	18.00%
Historical Claims Duration, Year Five	19.80%
Historical Claims Duration, Year Six	10.10%
Historical Claims Duration, Year Seven	4.40%
Historical Claims Duration, Year Eight	8.50%