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Fair Value Measurements (Summary of Fair Value of Derivative Assets and Liabilities and Effect of Counterparty Netting) (Details) - USD ($)
$ in Millions
Jun. 30, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Net carrying value presented on balance sheet, floating-rate debt $ 90 $ 50
Difference In carrying value and fair value liability (909) 195
Net carrying value presented on balance sheet, fixed-rate debt, excluding capital leases [1] 8,584 8,629
Fixed Rate Debt Excluding Capital Leases [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Difference In carrying value and fair value liability [1] (909) 195
Carrying (Reported) Amount, Fair Value Disclosure [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Net carrying value presented on balance sheet, commodity derivative assets and investments 156 194
Net carrying value presented on balance sheet, commodity derivative liabilities and debt 8,706 8,708
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Rabbi Trust Assets [Member] | Investments And Long-term Receivables [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Rabbi trust assets 94 83
Exchange-cleared Instruments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash Collateral Received of Paid Not Offset on Balance Sheet 0  
Exchange-cleared Instruments [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Net carrying value presented on balance sheet, commodity derivative assets 6 65
Net carrying value presented on balance sheet, commodity derivative liabilities   2
OTC Instruments [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Net carrying value presented on balance sheet, commodity derivative assets 5 1
Net carrying value presented on balance sheet, commodity derivative liabilities 2 5
Physical Forward Contracts [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Net carrying value presented on balance sheet, commodity derivative assets [2] 51 45
Net carrying value presented on balance sheet, commodity derivative liabilities [2] 14 22
Fair Value, Measurements, Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Effect of counterparty netting, commodity derivative assets (570) (2,401)
Effect of collateral netting, commodity derivative assets   (100)
Total assets, fair value disclosure gross 726 2,695
Effect of counterparty netting, commodity derivative liabilities (570) (2,401)
Effect of collateral netting, commodity derivative liabilities (68)  
Total liabilities, fair value disclosure gross 10,253 10,914
Fair Value, Measurements, Recurring [Member] | Floating Rate Debt [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt, fair value gross 90 50
Fair Value, Measurements, Recurring [Member] | Fixed Rate Debt Excluding Capital Leases [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt, fair value gross [1] 9,493 8,434
Fair Value, Measurements, Recurring [Member] | Rabbi Trust Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Rabbi trust assets 94 83
Fair Value, Measurements, Recurring [Member] | Exchange-cleared Instruments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 574 2,554
Effect of counterparty netting, commodity derivative assets (568) (2,389)
Effect of collateral netting, commodity derivative assets   (100)
Commodity derivative liabilities, fair value gross 636 2,391
Effect of counterparty netting, commodity derivative liabilities (568) (2,389)
Effect of collateral netting, commodity derivative liabilities (68)  
Fair Value, Measurements, Recurring [Member] | OTC Instruments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 7 13
Effect of counterparty netting, commodity derivative assets (2) (12)
Commodity derivative liabilities, fair value gross 4 17
Effect of counterparty netting, commodity derivative liabilities (2) (12)
Fair Value, Measurements, Recurring [Member] | Physical Forward Contracts [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross [2] 51 45
Commodity derivative liabilities, fair value gross [2] 14 22
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets, fair value disclosure gross 352 1,937
Total liabilities, fair value disclosure gross 367 1,795
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Floating Rate Debt [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt, fair value gross 90 50
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Rabbi Trust Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Rabbi trust assets 94 83
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Exchange-cleared Instruments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 258 1,851
Commodity derivative liabilities, fair value gross 277 1,745
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Physical Forward Contracts [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross [2]   3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets, fair value disclosure gross 373 756
Total liabilities, fair value disclosure gross 9,885 9,119
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Fixed Rate Debt Excluding Capital Leases [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt, fair value gross [1] 9,493 8,434
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Exchange-cleared Instruments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 316 703
Commodity derivative liabilities, fair value gross 359 646
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | OTC Instruments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 7 13
Commodity derivative liabilities, fair value gross 4 17
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Physical Forward Contracts [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross [2] 50 40
Commodity derivative liabilities, fair value gross [2] 13 22
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets, fair value disclosure gross 1 2
Total liabilities, fair value disclosure gross 1  
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Physical Forward Contracts [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross [2] 1 $ 2
Commodity derivative liabilities, fair value gross [2] 1  
Interest Rate Contract [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Net carrying value presented on balance sheet, commodity derivative liabilities 16  
Interest Rate Contract [Member] | Fair Value, Measurements, Recurring [Member] | Derivative Financial Instruments, Liabilities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Interest-rate derivatives 16  
Interest Rate Contract [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Derivative Financial Instruments, Liabilities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Interest-rate derivatives $ 16  
[1] We carry fixed-rate debt on the balance sheet at amortized cost.
[2] Physical forward contracts may have a larger value on the balance sheet than disclosed in the fair value hierarchy when the remaining contract term at the reporting date is greater than 12 months and the short-term portion is an asset while the long-term portion is a liability, or vice versa.