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Segment reporting - Broker Concentration (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
Gross Premiums Written                      
Revenue, Major Customer [Line Items]                      
Gross premiums written                 $ 631,846 $ 578,252 $ 641,620
Concentration risk, percentage                 100.00% 100.00% 100.00%
Gross Premiums Written | Customer Concentration Risk                      
Revenue, Major Customer [Line Items]                      
Gross premiums written                 $ 631,846 $ 578,252 $ 641,620
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                 $ 160,404 $ 198,251 $ 243,581
Concentration risk, percentage                 25.40% 34.30% 38.00%
Gross Premiums Written | Customer Concentration Risk | Second largest broker                      
Revenue, Major Customer [Line Items]                      
Gross premiums written                 $ 150,377 $ 157,542 $ 128,648
Concentration risk, percentage                 23.80% 27.20% 20.10%
Gross Premiums Written | Customer Concentration Risk | Third largest broker                      
Revenue, Major Customer [Line Items]                      
Gross premiums written                 $ 137,685 $ 70,524 $ 107,612
Concentration risk, percentage                 21.80% 12.20% 16.80%
Gross Premiums Written | Customer Concentration Risk | Other                      
Revenue, Major Customer [Line Items]                      
Gross premiums written                 $ 183,380 $ 151,935 $ 161,779
Concentration risk, percentage                 29.00% 26.30% 25.10%