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Derivative and Hedging Activities - Fair Value Level 3 Measurements Table (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]    
Derivative assets (liabilities), at fair value, net, beginning of period $ (35)  
Derivative assets (liabilities), at fair value, net, end of period (318)  
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]    
Derivative assets (liabilities), at fair value, net, beginning of period (15)  
Derivative assets (liabilities), at fair value, net, end of period (45)  
Fair Value, Inputs, Level 3 [Member] | Oil, Natural Gas and NGL Sales [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]    
Gains (loses) included in earnings (32) $ 19
Change in unrealized gains (losses) related to assets still held at reporting date (30) 12
Fair Value, Inputs, Level 3 [Member] | Commodity contract [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]    
Derivative assets (liabilities), at fair value, net, beginning of period (15) (10)
Gains (loses) included in earnings (32) 19
Settlements 2 1
Derivative assets (liabilities), at fair value, net, end of period $ (45) $ 10