Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements (Schedule of Derivative Instruments) (Details)

v3.7.0.1
Fair Value Measurements (Schedule of Derivative Instruments) (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Mar. 31, 2016
Feb. 29, 2016
Liabilities:          
Asset retirement obligation capitalized $ 9,505 $ 3,195      
Level 1          
Assets:          
Derivative Instruments 0   $ 0    
Liabilities:          
Derivative Instruments 0   0    
Level 2          
Assets:          
Derivative Instruments 66,177   9,184    
Liabilities:          
Derivative Instruments 36,304   145,978    
Level 3          
Assets:          
Derivative Instruments 0   0    
Liabilities:          
Derivative Instruments 0   $ 0    
Asset retirement obligation capitalized $ 9,500        
Level 3 | Investment in Grizzly Oil Sands ULC          
Liabilities:          
Equity investment       $ 39,100  
Level 3 | Investment in Strike Force Midstream LLC          
Liabilities:          
Equity investment         $ 22,500