Use of Collateral (Details) - USD ($) $ in Millions |
Dec. 31, 2016 |
Dec. 31, 2015 |
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|---|---|---|---|---|---|---|---|---|---|---|---|
| Financial Instruments Owned and Pledged as Collateral [Line Items] | |||||||||||
| Margin Deposit Assets | [1] | $ 350 | $ 89 | ||||||||
| Natural gas and power prepayments | 25 | 34 | |||||||||
| Total margin deposits and natural gas and power prepayments with our counterparties | [2] | 375 | 123 | ||||||||
| Letters of credit issued | 798 | 600 | |||||||||
| First priority liens under power and natural gas agreements | [3] | 206 | 382 | ||||||||
| First priority liens under interest rate hedging instruments | 55 | 92 | |||||||||
| Letters of Credit Issued and First Priority Liens Under Power Natural Gas And Interest Rate Hedging Instruments | 1,059 | 1,074 | |||||||||
| Margin deposits posted with us by our counterparties | [1],[4] | 16 | 35 | ||||||||
| Letters of credit posted with us by our counterparties | 43 | 24 | |||||||||
| Total margin deposits and letters of credit posted with us by our counterparties | 59 | 59 | |||||||||
| Use of Collateral (Textuals) [Abstract] | |||||||||||
| Margin And Prepayment Amounts Included In Margin Deposits And Other Prepaid Expenses | 366 | 101 | |||||||||
| Margin And Prepayment Amounts Included In Other Assets | 9 | 22 | |||||||||
| Champion Energy [Member] | |||||||||||
| Financial Instruments Owned and Pledged as Collateral [Line Items] | |||||||||||
| First priority liens under power and natural gas agreements | [3] | $ 185 | $ 345 | ||||||||
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