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Stock option and award plan: Incentive award plan (Details) - USD ($)
$ in Millions
1 Months Ended 12 Months Ended
May 31, 2016
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Incentive award plan        
Granted (in shares) 73,450,000      
Fair value of shares $ 2.9      
Stock options        
Incentive award plan        
Vesting period   4 years    
Expiration period   10 years    
Exercise period of options vested, when an employee is terminated prior to full vesting   90 days    
Restricted stock        
Incentive award plan        
Granted (in shares)   358,130 100,000 800,000
Fair value of shares   $ 12.0 $ 14.3 $ 19.0
Minimum | Restricted stock        
Incentive award plan        
Vesting period   3 years    
Maximum | Restricted stock        
Incentive award plan        
Vesting period   4 years