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Risk Management Activities and Derivative Transactions Offsetting for Derivative Liabilities (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Sep. 30, 2017
--- | --- | ---
Offsetting Liabilities [Line Items] |  |  |
Total derivatives not subject to master netting or similar arrangement | [1] | $ 16 |
Gross amounts of recognized liabilities |  | 1,520 | $ 1,619
Gross Amounts Offset in the Balance Sheet | [2] | 0 | (4)
Net Amounts of Liabilities Presented in the Balance Sheet | [1] | 1,613 | 1,685
Commodity derivatives not subject to master netting or similar arrangement | [1] | 93 | 70
Total |  | 1,613 | 1,689
Forward Contract Derivative Asset, at Fair Value |  | 37 | 19
Currency Swap |  |  |
Offsetting Liabilities [Line Items] |  |  |
Gross amounts of recognized liabilities | [3] | 66 | 103
Gross Amounts Offset in the Balance Sheet | [2],[3] | 0 | 0
Net Amounts of Liabilities Presented in the Balance Sheet | [1],[3] | 66 | 103
Interest Rate Contract |  |  |
Offsetting Liabilities [Line Items] |  |  |
Gross amounts of recognized liabilities | [3] | 1,454 | 1,511
Gross Amounts Offset in the Balance Sheet | [2],[3] | 0 | 0
Net Amounts of Liabilities Presented in the Balance Sheet | [1],[3] | 1,454 | 1,511
Commodity derivatives under FTP |  |  |
Offsetting Liabilities [Line Items] |  |  |
Gross amounts of recognized liabilities |  |  | 5
Gross Amounts Offset in the Balance Sheet | [2] |  | (4)
Net Amounts of Liabilities Presented in the Balance Sheet | [1] |  | 1
Total derivatives subject to master netting or similar arrangement |  |  |
Offsetting Liabilities [Line Items] |  |  |
Net Amounts of Liabilities Presented in the Balance Sheet | [1] | 1,520 | 1,615
Letter of Credit |  |  |
Offsetting Liabilities [Line Items] |  |  |
Amount of letters of credit outstanding |  | 1,000 | 1,200
Fair Value, Inputs, Level 2 |  |  |
Offsetting Liabilities [Line Items] |  |  |
Forward Contract Derivative Asset, at Fair Value |  | $ 37 | $ 19
[1] | There are no derivative contracts subject to a master netting arrangement or similar agreement that are not offset in the consolidated balance sheets.
[2] | Amounts primarily include counterparty netting of derivative contracts, margin account deposits for futures commission merchants transactions, and cash collateral received or paid in accordance with the accounting guidance for derivatives and hedging transactions.
[3] | Letters of credit of approximately $1.0 billion and $1.2 billion were posted as collateral at December 31, 2017, and September 30, 2017, respectively, to partially secure the liability positions of one of the currency swaps and one of the interest rate swaps in accordance with the collateral requirements for these derivatives.

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