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Stockholders equity and dividend payment - Dividend payment (Details) - USD ($)
 3 Months Ended 12 Months Ended
Dec. 06, 2017
Aug. 31, 2017
May 31, 2017
Feb. 22, 2017
Nov. 23, 2016
Aug. 31, 2016
May 25, 2016
Feb. 24, 2016
Nov. 25, 2015
Aug. 20, 2015
May 22, 2015
Feb. 19, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dividend payment                              
Total payment                         $ 23,300,000 $ 66,400,000 $ 49,200,000
Per share Common                         $ 0.20 $ 0.71 $ 0.53
Quarterly dividend [Member]                              
Dividend payment                              
Payment $ 2,800,000 $ 2,800,000 $ 10,100,000 $ 7,600,000 $ 1,900,000 $ 21,500,000 $ 23,300,000 $ 19,700,000 $ 16,700,000 $ 13,900,000 $ 13,900,000 $ 4,600,000      
Per share Common $ 0.02 $ 0.02 $ 0.08 $ 0.08 $ 0.02 $ 0.23 $ 0.25 $ 0.21 $ 0.18 $ 0.15 $ 0.15 $ 0.05