Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements - Narrative (Details)

v3.10.0.1
Fair Value Measurements - Narrative (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Mar. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Asset retirement obligation capitalized $ 1,827 $ 16,300  
Asset retirement obligation, revised 375 $ (25,970)  
Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Asset retirement obligation capitalized 1,800    
Asset retirement obligation, revised $ 400    
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