Segment reporting - Concentration Risk (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenue, Major Customer [Line Items]
Assumed Premiums Written	$ 157,571	$ 82,637	$ 361,702	$ 402,228
Gross Premiums As Percentage Of Total Gross Premiums	100.00%	100.00%	100.00%	100.00%
Property and Casualty Reinsurance | Property
Revenue, Major Customer [Line Items]
Assumed Premiums Written	$ 85,451	$ 41,862	$ 131,824	$ 95,843
Gross Premiums As Percentage Of Total Gross Premiums	54.20%	50.70%	36.40%	23.80%
Property and Casualty Reinsurance | Casualty
Revenue, Major Customer [Line Items]
Assumed Premiums Written	$ 27,143	$ 24,438	$ 94,083	$ 102,330
Gross Premiums As Percentage Of Total Gross Premiums	17.20%	29.60%	26.00%	25.40%
Property and Casualty Reinsurance | Specialty
Revenue, Major Customer [Line Items]
Assumed Premiums Written	$ 44,977	$ 16,337	$ 135,795	$ 209,408
Gross Premiums As Percentage Of Total Gross Premiums	28.60%	19.70%	37.60%	52.10%
Property and Casualty Reinsurance | Prospective Reinsurance Contract
Revenue, Major Customer [Line Items]
Assumed Premiums Written	$ 157,571	$ 82,637	$ 361,702	$ 407,581
Gross Premiums As Percentage Of Total Gross Premiums	100.00%	100.00%	100.00%	101.30%
Property and Casualty Reinsurance | Retroactive Reinsurance Contract
Revenue, Major Customer [Line Items]
Assumed Premiums Written	$ 0	$ 0	$ 0	$ (5,353)
Gross Premiums As Percentage Of Total Gross Premiums	0.00%	0.00%	0.00%	(1.30%)