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Description of the business and recent developments: Financial Instruments (Details) - USD ($)
$ in Millions
Dec. 31, 2015
Dec. 31, 2014
Financial instruments    
Letters of credit, outstanding amount $ 0.3 $ 0.4
Senior Unsecured Notes | Level 2    
Financial instruments    
Debt instrument, fair value amount 187.5  
Senior Secured Notes    
Financial instruments    
Debt instrument, fair value amount 250.0  
Senior Secured Notes | Level 2    
Financial instruments    
Debt instrument, fair value amount $ 244.1