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Segment reporting - Geographic information (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Revenue from External Customer [Line Items]      
Gross premiums written $ 569,501 $ 668,414 $ 650,187
Gross Premiums Written      
Revenue from External Customer [Line Items]      
Gross premiums written $ 588,540 $ 668,414 $ 650,187
Concentration risk, percentage 100.00% 100.00% 100.00%
Gross Premiums Written | United States      
Revenue from External Customer [Line Items]      
Gross premiums written $ 358,582 $ 361,511 $ 409,584
Concentration risk, percentage 60.90% 54.10% 63.00%
Gross Premiums Written | United Kingdom      
Revenue from External Customer [Line Items]      
Gross premiums written $ 110,576 $ 161,638 $ 60,623
Concentration risk, percentage 18.80% 24.20% 9.30%
Gross Premiums Written | Bermuda      
Revenue from External Customer [Line Items]      
Gross premiums written $ 108,598 $ 137,278 $ 86,236
Concentration risk, percentage 18.50% 20.50% 13.30%
Gross Premiums Written | Other      
Revenue from External Customer [Line Items]      
Gross premiums written $ 10,784 $ 7,987 $ 93,744
Concentration risk, percentage 1.80% 1.20% 14.40%