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Accounting for Derivative Instruments and Hedging Activities - Offsetting of derivatives by counterparty master agreement level and collateral received (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid    
Derivative assets $ 1,369 $ 798
Cash Collateral (Held) / Posted (240) (37)
Gross Amounts of Recognized Derivative Liabilities (1,054) (752)
Cash Collateral Posted 211 171
Gross Amounts of Recognized Assets / Liabilities 315 46
Derivative Instruments 0 0
Cash Collateral (Held) / Posted (81) 62
Net Amount 234 108
Commodity contracts    
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid    
Derivative assets 1,277 745
Derivative Instruments (835) (578)
Cash Collateral (Held) / Posted (201) (11)
Net Amount 241 156
Gross Amounts of Recognized Derivative Liabilities (1,024) (693)
Derivative Instruments 835 578
Cash Collateral Posted 120 73
Net Amount (69) (42)
Gross Amounts of Recognized Assets / Liabilities 253 52
Derivative Instruments 0 0
Cash Collateral (Held) / Posted (81) 62
Net Amount 172 114
Interest rate contracts    
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid    
Derivative assets 92 53
Derivative Instruments (4) (3)
Cash Collateral (Held) / Posted 0 0
Net Amount 88 50
Gross Amounts of Recognized Derivative Liabilities (30) (59)
Derivative Instruments 4 3
Cash Collateral Posted 0 0
Net Amount (26) (56)
Gross Amounts of Recognized Assets / Liabilities 62 (6)
Derivative Instruments 0 0
Cash Collateral (Held) / Posted 0 0
Net Amount $ 62 $ (6)