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Stockholders' equity and dividend payment, Dividend payment (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Feb. 25, 2020
Nov. 14, 2019
Aug. 29, 2019
May 28, 2019
Feb. 26, 2019
Nov. 23, 2018
Aug. 31, 2018
May 30, 2018
Feb. 28, 2018
Dec. 06, 2017
Aug. 31, 2017
May 31, 2017
Feb. 22, 2017
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dividend Payment [Abstract]                                
Total payment   $ 7,300 $ 2,800 $ 11,400 $ 7,100 $ 2,900 $ 2,900 $ 2,900 $ 2,900 $ 2,800 $ 2,800 $ 10,100 $ 7,600 $ 28,685 $ 11,487 $ 23,328
Per share Common (in dollars per share)   $ 0.05 $ 0.02 $ 0.08 $ 0.05 $ 0.02 $ 0.02 $ 0.02 $ 0.02 $ 0.02 $ 0.02 $ 0.08 $ 0.08 $ 0.20 $ 0.08 $ 0.20
Subsequent events [Member]                                
Dividend Payment [Abstract]                                
Per share Common (in dollars per share) $ 0.32                              
Dividend paid, record date Feb. 18, 2020                              
Payment $ 47,000