Segment reporting - Concentration Risk (Details) - Gross Premiums Written	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue, Major Customer [Line Items]
Concentration risk, percentage	100.00%	100.00%	100.00%
Contract Concentration Risk
Revenue, Major Customer [Line Items]
Concentration risk, percentage	100.00%	100.00%	100.00%
Contract Concentration Risk | Contract 1
Revenue, Major Customer [Line Items]
Concentration risk, percentage	15.20%	17.50%	16.10%
Contract Concentration Risk | Contract 2
Revenue, Major Customer [Line Items]
Concentration risk, percentage		12.10%	14.10%
Contract Concentration Risk | Contract 3
Revenue, Major Customer [Line Items]
Concentration risk, percentage			13.10%
Contract Concentration Risk | Total for contracts contributing greater than 10% each
Revenue, Major Customer [Line Items]
Concentration risk, percentage	15.20%	29.60%	43.30%
Contract Concentration Risk | Total for contracts contributing less than 10% each
Revenue, Major Customer [Line Items]
Concentration risk, percentage	84.80%	70.40%	56.70%