Quarterly report pursuant to Section 13 or 15(d)

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

v3.8.0.1
Fair Value Measurements (Schedule of Derivative Instruments) (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Mar. 31, 2016
Feb. 01, 2016
Liabilities          
Asset retirement obligation capitalized $ 11,557 $ 6,726      
Level 1          
Assets          
Derivative Instruments 0   $ 0    
Liabilities          
Derivative Instruments 0   0    
Level 2          
Assets          
Derivative Instruments 41,741   9,184    
Liabilities          
Derivative Instruments 48,842   145,978    
Level 3          
Assets          
Derivative Instruments 0   0    
Liabilities          
Derivative Instruments $ 0   $ 0    
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