Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements (Details)

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Fair Value Measurements (Details) - USD ($)
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets $ 142,794,000 $ 78,391,000  
Asset retirement obligation capitalized 8,800,000 9,295,000 $ 3,556,000
Equity investments 242,393,000 369,581,000  
Level 1      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Liabilities 0    
Level 1 | Derivative Instruments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets 0 0  
Level 2      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Liabilities 7,372,000    
Level 2 | Derivative Instruments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets 193,882,000 102,839,000  
Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Liabilities 0    
Asset retirement obligation capitalized 8,800,000    
Level 3 | Derivative Instruments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets 0 0  
Investment in Grizzly Oil Sands ULC      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity investments $ 50,645,000 $ 180,218,000