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Segment reporting - Business Line (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Revenue, Major Customer [Line Items]                      
Gross premiums written $ 134,230 $ 95,388 $ 82,637 $ 319,591 $ 120,063 $ 30,064 $ 49,765 $ 378,360 $ 631,846 $ 578,252 $ 641,620
Property and Casualty Reinsurance                      
Revenue, Major Customer [Line Items]                      
Gross premiums written                 $ 631,846 $ 578,252 $ 641,620
Gross Premiums As Percentage Of Total Gross Premiums                 100.00% 100.00% 100.00%
Property and Casualty Reinsurance | Property                      
Revenue, Major Customer [Line Items]                      
Gross premiums written                 $ 144,271 $ 9,070 $ 136,999
Gross Premiums As Percentage Of Total Gross Premiums                 22.80% 1.60% 21.40%
Property and Casualty Reinsurance | Casualty                      
Revenue, Major Customer [Line Items]                      
Gross premiums written                 $ 151,893 $ 235,789 $ 269,759
Gross Premiums As Percentage Of Total Gross Premiums                 24.00% 40.80% 42.00%
Property and Casualty Reinsurance | Specialty                      
Revenue, Major Customer [Line Items]                      
Gross premiums written                 $ 248,044 $ 259,173 $ 125,511
Gross Premiums As Percentage Of Total Gross Premiums                 39.30% 44.80% 19.60%
Property and Casualty Reinsurance | Total prospective reinsurance contracts                      
Revenue, Major Customer [Line Items]                      
Gross premiums written                 $ 544,208 $ 504,032 $ 532,269
Gross Premiums As Percentage Of Total Gross Premiums                 86.10% 87.20% 83.00%
Property and Casualty Reinsurance | Retroactive reinsurance contracts                      
Revenue, Major Customer [Line Items]                      
Gross premiums written                 $ 87,638 $ 74,220 $ 109,351
Gross Premiums As Percentage Of Total Gross Premiums                 13.90% 12.80% 17.00%