Fair Value Measurements - Assets and Liabilities Accounted for at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cash collateral netted with derivative liabilities $ 39 $ 8
Embedded derivatives in commodity contracts    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Embedded derivatives, current 11 12
Fair Value, Measurements, Recurring    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Commodity derivative instruments, assets - netting and collateral (21) (118)
Commodity derivative instruments, assets - collateral pledged not offset 22 8
Other assets 3 3
Total assets at fair value 3 12
Commodity derivative instruments, liabilities - netting and collateral (60) (126)
Commodity derivative instruments, liabilities - collateral pledged not offset 0 0
Total liabilities at fair value 60 66
Fair Value, Measurements, Recurring | Commodity derivatives    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Commodity derivative instruments, assets - netting and collateral [1] (21) (118)
Commodity derivative instruments, assets - net carrying value on balance sheet [2] 0 9
Commodity derivative instruments, assets - collateral pledged not offset 22 8
Commodity derivative instruments, liabilities - netting and collateral [1] (60) (126)
Commodity derivative instruments, liabilities - net carrying value on balance sheet [2] 1 2
Commodity derivative instruments, liabilities - collateral pledged not offset 0 0
Fair Value, Measurements, Recurring | Embedded derivatives in commodity contracts    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Commodity derivative instruments, liabilities - netting and collateral 0 0
Commodity derivative instruments, liabilities - net carrying value on balance sheet [2] 59 64
Commodity derivative instruments, liabilities - collateral pledged not offset 0 0
Fair Value, Measurements, Recurring | Level 1    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Other assets 3 3
Total assets at fair value 24 130
Total liabilities at fair value 60 126
Fair Value, Measurements, Recurring | Level 1 | Commodity derivatives    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Commodity derivative instruments, assets - gross 21 127
Commodity derivative instruments, liabilities - gross 60 126
Fair Value, Measurements, Recurring | Level 1 | Embedded derivatives in commodity contracts    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Commodity derivative instruments, liabilities - gross 0 0
Fair Value, Measurements, Recurring | Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Other assets 0 0
Total assets at fair value 0 0
Total liabilities at fair value 0 0
Fair Value, Measurements, Recurring | Level 2 | Commodity derivatives    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Commodity derivative instruments, assets - gross 0 0
Commodity derivative instruments, liabilities - gross 0 0
Fair Value, Measurements, Recurring | Level 2 | Embedded derivatives in commodity contracts    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Commodity derivative instruments, liabilities - gross 0 0
Fair Value, Measurements, Recurring | Level 3    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Other assets 0 0
Total assets at fair value 0 0
Total liabilities at fair value 60 66
Fair Value, Measurements, Recurring | Level 3 | Commodity derivatives    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Commodity derivative instruments, assets - gross 0 0
Commodity derivative instruments, liabilities - gross 1 [3] 2
Fair Value, Measurements, Recurring | Level 3 | Embedded derivatives in commodity contracts    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Commodity derivative instruments, liabilities - gross [3] $ 59 $ 64
[1] Represents the impact of netting assets, liabilities and cash collateral when a legal right of offset exists. As of March 31, 2018, cash collateral of $39 million was netted with the mark-to-market derivative liabilities. As of December 31, 2017, $8 million was netted with mark-to-market derivative liabilities.
[2] We have no derivative contracts that are subject to master netting arrangements reflected gross on the balance sheet.
[3] Level 3 includes $11 million and $12 million classified as current at March 31, 2018 and December 31, 2017, respectively.