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Segment reporting - Geographic information (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 from External 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
Gross Premiums Written                      
Revenue from External Customer [Line Items]                      
Gross premiums written                 $ 631,846 $ 578,252 $ 641,620
Concentration risk, percentage                 100.00% 100.00% 100.00%
Gross Premiums Written | United States                      
Revenue from External Customer [Line Items]                      
Gross premiums written                 $ 326,987 $ 326,261 $ 352,539
Concentration risk, percentage                 51.80% 56.40% 54.90%
Gross Premiums Written | United Kingdom                      
Revenue from External Customer [Line Items]                      
Gross premiums written                 $ 154,909 $ 63,619 $ 203,768
Concentration risk, percentage                 24.50% 11.00% 31.80%
Gross Premiums Written | Bermuda                      
Revenue from External Customer [Line Items]                      
Gross premiums written                 $ 142,246 $ 93,406 $ 62,234
Concentration risk, percentage                 22.50% 16.20% 9.70%
Gross Premiums Written | Other                      
Revenue from External Customer [Line Items]                      
Gross premiums written                 $ 7,704 $ 94,966 $ 23,079
Concentration risk, percentage                 1.20% 16.40% 3.60%