Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements - Narrative (Details)

v3.8.0.1
Fair Value Measurements - Narrative (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Mar. 31, 2016
Feb. 01, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Asset retirement obligation capitalized $ 42,270 $ 10,971 $ 8,800    
Asset retirement obligation, revised   $ 0      
Level 3          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Asset retirement obligation capitalized 16,300        
Asset retirement obligation, revised $ 25,970        
Level 3 | Investment in Grizzly Oil Sands ULC          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Fair value of equity investment       $ 39,100  
Level 3 | Strike Force Midstream LLC          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Fair value of equity investment         $ 22,500