XML 71 R55.htm IDEA: XBRL DOCUMENT v3.19.1
Stockholders' equity and dividend payment - Dividend payment (Details) - USD ($)
 3 Months Ended 12 Months Ended
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
Nov. 23, 2016
Aug. 31, 2016
May 25, 2016
Feb. 24, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dividend payment                                
Payment $ 7,100,000                              
Total payment                           $ 11,500,000 $ 23,300,000 $ 66,400,000
Per share Common $ 0.05                         $ 0.08 $ 0.20 $ 0.71
Quarterly dividend [Member]                                
Dividend payment                                
Payment   $ 2,900,000 $ 2,900,000 $ 2,900,000 $ 2,900,000 $ 2,800,000 $ 2,800,000 $ 10,100,000 $ 7,600,000 $ 1,900,000 $ 21,500,000 $ 23,300,000 $ 19,700,000      
Per share Common   $ 0.02 $ 0.02 $ 0.02 $ 0.02 $ 0.02 $ 0.02 $ 0.08 $ 0.08 $ 0.02 $ 0.23 $ 0.25 $ 0.21