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Stock option and award plan: Incentive award plan (Details) - USD ($)
$ in Millions
1 Months Ended 3 Months Ended 12 Months Ended
Feb. 28, 2018
May 31, 2017
May 31, 2016
Jun. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Incentive award plan              
Share increase (in shares)   1,200,000          
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    
Granted (in shares)       470,404      
Fair value of shares       $ 19.0      
Restricted stock              
Incentive award plan              
Granted (in shares)     73,450   499,654 400,000 100,000
Fair value of shares     $ 2.9   $ 12.6 $ 12.0 $ 14.3
Restricted stock | Minimum              
Incentive award plan              
Vesting period         3 years    
Restricted stock | Maximum              
Incentive award plan              
Vesting period         4 years    
Performance conditions | Executives              
Incentive award plan              
Shares subject to conditions (in shares)       24,050      
Performance conditions | CEO              
Incentive award plan              
Granted (in shares) 105,000            
Shares subject to conditions (in shares)       105,000