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Segment reporting - Broker Concentration (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Revenue, Major Customer [Line Items]                      
Gross premiums written $ 190,358 $ 124,737 $ 147,740 $ 125,705 $ 191,203 $ 184,856 $ 149,035 $ 143,320 $ 588,540 $ 668,414 $ 650,187
Gross Premiums Written                      
Revenue, Major Customer [Line Items]                      
Concentration risk, percentage                 100.00% 100.00% 100.00%
Gross Premiums Written | Customer Concentration Risk                      
Revenue, Major Customer [Line Items]                      
Gross premiums written                 $ 588,540 $ 668,414 $ 650,187
Concentration risk, percentage                 100.00% 100.00% 100.00%
Gross Premiums Written | Customer Concentration Risk | Largest broker                      
Revenue, Major Customer [Line Items]                      
Gross premiums written                 $ 189,097 $ 197,100 $ 224,198
Concentration risk, percentage                 32.10% 29.50% 34.50%
Gross Premiums Written | Customer Concentration Risk | Second largest broker                      
Revenue, Major Customer [Line Items]                      
Gross premiums written                 $ 164,569 $ 144,611 $ 163,336
Concentration risk, percentage                 28.00% 21.60% 25.10%
Gross Premiums Written | Customer Concentration Risk | Third largest broker                      
Revenue, Major Customer [Line Items]                      
Gross premiums written                 $ 67,550 $ 140,197 $ 120,381
Concentration risk, percentage                 11.50% 21.00% 18.50%
Gross Premiums Written | Customer Concentration Risk | Fourth largest broker                      
Revenue, Major Customer [Line Items]                      
Gross premiums written                     $ 70,523
Concentration risk, percentage                     10.80%
Gross Premiums Written | Customer Concentration Risk | Other                      
Revenue, Major Customer [Line Items]                      
Gross premiums written                 $ 167,324 $ 186,506 $ 71,749
Concentration risk, percentage                 28.40% 27.90% 11.10%