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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
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Difference in Carrying Value and Fair Value $ (570) $ 195
Floating-rate debt 260 50
Fixed-rate debt, excluding capital leases 9,690 8,629
Fixed-rate debt, excluding capital leases    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Difference in Carrying Value and Fair Value (570) 195
Net Carrying Value Presented on the Balance Sheet    
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 222 194
Net carrying value presented on balance sheet, commodity derivative liabilities and debt 10,016 8,708
Net Carrying Value Presented on the Balance Sheet | Rabbi trust assets | Equity investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Rabbi trust assets 97 83
Exchange-cleared instruments | Net Carrying Value Presented on the Balance Sheet    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Net carrying value presented on balance sheet, commodity derivative assets 16 65
Net carrying value presented on balance sheet, commodity derivative liabilities 0 2
OTC instruments | Net Carrying Value Presented on the Balance Sheet    
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 0 5
Physical forward contracts | Net Carrying Value Presented on the Balance Sheet    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Net carrying value presented on balance sheet, commodity derivative assets 96 45
Net carrying value presented on balance sheet, commodity derivative liabilities 66 22
Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Effect of counterparty netting, commodity derivative assets (629) (2,401)
Effect of collateral netting, commodity derivative assets 0 (100)
Total assets, fair value disclosure gross 851 2,695
Effect of counterparty netting, commodity derivative liabilities (629) (2,401)
Effect of Collateral Netting (73)  
Total liabilities, fair value disclosure gross 11,288 10,914
Fair Value, Measurements, Recurring | Floating-rate debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt excluding capital leases, fair value gross 260 50
Fair Value, Measurements, Recurring | Fixed-rate debt, excluding capital leases    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt excluding capital leases, fair value gross 10,260 8,434
Fair Value, Measurements, Recurring | Rabbi trust assets    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Rabbi trust assets 97 83
Fair Value, Measurements, Recurring | Exchange-cleared instruments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 644 2,554
Effect of counterparty netting, commodity derivative assets (628) (2,389)
Effect of collateral netting, commodity derivative assets 0 (100)
Commodity derivative liabilities, fair value gross 701 2,391
Effect of counterparty netting, commodity derivative liabilities (628) (2,389)
Effect of Collateral Netting (73)  
Fair Value, Measurements, Recurring | OTC instruments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 6 13
Effect of counterparty netting, commodity derivative assets (1) (12)
Commodity derivative liabilities, fair value gross 1 17
Effect of counterparty netting, commodity derivative liabilities (1) (12)
Fair Value, Measurements, Recurring | Physical forward contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 96 45
Effect of counterparty netting, commodity derivative assets 0 0
Commodity derivative liabilities, fair value gross 66 22
Effect of counterparty netting, commodity derivative liabilities 0 0
Fair Value, Measurements, Recurring | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets, fair value disclosure gross 370 1,937
Total liabilities, fair value disclosure gross 299 1,795
Fair Value, Measurements, Recurring | Level 1 | Floating-rate debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt excluding capital leases, fair value gross 50 50
Fair Value, Measurements, Recurring | Level 1 | Fixed-rate debt, excluding capital leases    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt excluding capital leases, fair value gross 0 0
Fair Value, Measurements, Recurring | Level 1 | Rabbi trust assets    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Rabbi trust assets 97 83
Fair Value, Measurements, Recurring | Level 1 | Exchange-cleared instruments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 273 1,851
Commodity derivative liabilities, fair value gross 249 1,745
Fair Value, Measurements, Recurring | Level 1 | OTC instruments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 0 0
Commodity derivative liabilities, fair value gross 0 0
Fair Value, Measurements, Recurring | Level 1 | Physical forward contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 0 3
Commodity derivative liabilities, fair value gross 0 0
Fair Value, Measurements, Recurring | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets, fair value disclosure gross 479 756
Total liabilities, fair value disclosure gross 10,984 9,119
Fair Value, Measurements, Recurring | Level 2 | Floating-rate debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt excluding capital leases, fair value gross 210 0
Fair Value, Measurements, Recurring | Level 2 | Fixed-rate debt, excluding capital leases    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt excluding capital leases, fair value gross 10,260 8,434
Fair Value, Measurements, Recurring | Level 2 | Rabbi trust assets    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Rabbi trust assets 0 0
Fair Value, Measurements, Recurring | Level 2 | Exchange-cleared instruments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 371 703
Commodity derivative liabilities, fair value gross 452 646
Fair Value, Measurements, Recurring | Level 2 | OTC instruments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 6 13
Commodity derivative liabilities, fair value gross 1 17
Fair Value, Measurements, Recurring | Level 2 | Physical forward contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 94 40
Commodity derivative liabilities, fair value gross 61 22
Fair Value, Measurements, Recurring | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets, fair value disclosure gross 2 2
Total liabilities, fair value disclosure gross 5 0
Fair Value, Measurements, Recurring | Level 3 | Floating-rate debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt excluding capital leases, fair value gross 0 0
Fair Value, Measurements, Recurring | Level 3 | Fixed-rate debt, excluding capital leases    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt excluding capital leases, fair value gross 0 0
Fair Value, Measurements, Recurring | Level 3 | Rabbi trust assets    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Rabbi trust assets 0 0
Fair Value, Measurements, Recurring | Level 3 | Exchange-cleared instruments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 0 0
Commodity derivative liabilities, fair value gross 0 0
Fair Value, Measurements, Recurring | Level 3 | OTC instruments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 0 0
Commodity derivative liabilities, fair value gross 0 0
Fair Value, Measurements, Recurring | Level 3 | Physical forward contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 2 2
Commodity derivative liabilities, fair value gross 5 $ 0
Derivative Financial Instruments, Assets | Interest Rate Contract | Net Carrying Value Presented on the Balance Sheet    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Net carrying value presented on balance sheet, commodity derivative assets 8  
Derivative Financial Instruments, Assets | Interest Rate Contract | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 8  
Effect of counterparty netting, commodity derivative assets 0  
Derivative Financial Instruments, Assets | Interest Rate Contract | Fair Value, Measurements, Recurring | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross 0  
Derivative Financial Instruments, Assets | Interest Rate Contract | Fair Value, Measurements, Recurring | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets, fair value gross $ 8