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Fair Value Measurements and Credit Concentration (Net Assets and Liabilities Measured on Recurring Basis Level 3 Significant Unobservable Inputs) (Details) - Recurring - Level 3 - USD ($) $ in Millions | Mar. 31, 2016 | Dec. 31, 2015
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Auction Rate Securities |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Asset, fair value net |  | $ 6 | [1] | $ 6
Auction Rate Securities | Discounted Cash Flow | Minimum |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Modeled from SIFMA Index | [2] | 45.00% |  | 46.00%
Auction Rate Securities | Discounted Cash Flow | Maximum |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Modeled from SIFMA Index | [2] | 47.00% |  | 47.00%
Auction Rate Securities | Discounted Cash Flow | Weighted Average |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Modeled from SIFMA Index | [2] | 46.00% |  | 46.50%
Natural gas contracts |  |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |  |
Derivative, fair value, net | [3] | $ 57 |  | $ 55
Natural gas contracts | Discounted Cash Flow | Minimum |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Proprietary model used to calculate forward prices | [2] | 11.00% |  | 10.00%
Natural gas contracts | Discounted Cash Flow | Maximum |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Proprietary model used to calculate forward prices | [2] | 100.00% |  | 100.00%
Natural gas contracts | Discounted Cash Flow | Weighted Average |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Proprietary model used to calculate forward prices | [2] | 57.00% |  | 50.00%
Power sales contracts |  |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |  |
Derivative, fair value, net | [4] | $ 16 |  | $ 13
Power sales contracts | Discounted Cash Flow | Minimum |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Proprietary model used to calculate forward prices | [2] | 21.00% |  | 10.00%
Power sales contracts | Discounted Cash Flow | Maximum |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Proprietary model used to calculate forward prices | [2] | 100.00% |  | 100.00%
Power sales contracts | Discounted Cash Flow | Weighted Average |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Proprietary model used to calculate forward prices | [2] | 69.00% |  | 100.00%
FTR purchase contracts |  |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |  |
Derivative, fair value, net | [5] | $ (2) |  | $ (2)
FTR purchase contracts | Discounted Cash Flow | Minimum |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Historical settled prices used to model forward prices | [2] | 100.00% |  | 100.00%
FTR purchase contracts | Discounted Cash Flow | Maximum |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Historical settled prices used to model forward prices | [2] | 100.00% |  | 100.00%
FTR purchase contracts | Discounted Cash Flow | Weighted Average |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Historical settled prices used to model forward prices | [2] | 100.00% |  | 100.00%
Heat rate call options |  |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |  |
Derivative, fair value, net | [6] | $ (1) |  | $ (10)
Heat rate call options | Discounted Cash Flow | Minimum |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Proprietary model used to calculate forward prices | [2] | 100.00% |  | 100.00%
Heat rate call options | Discounted Cash Flow | Maximum |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Proprietary model used to calculate forward prices | [2] | 100.00% |  | 100.00%
Heat rate call options | Discounted Cash Flow | Weighted Average |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Proprietary model used to calculate forward prices | [2] | 100.00% |  | 100.00%
CRR purchase contracts |  |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |  |
Derivative, fair value, net | [5] | $ (2) |  | $ (2)
CRR purchase contracts | Discounted Cash Flow | Minimum |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Proprietary model used to calculate forward prices | [2] | 100.00% |  | 100.00%
CRR purchase contracts | Discounted Cash Flow | Maximum |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Proprietary model used to calculate forward prices | [2] | 100.00% |  | 100.00%
CRR purchase contracts | Discounted Cash Flow | Weighted Average |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |
Proprietary model used to calculate forward prices | [2] | 100.00% |  | 100.00%
[1] | The model used to calculate fair value incorporates an assumption that the auctions will continue to fail. As the modeled forward rates of the SIFMA Index increase/(decrease), the fair value of the securities increases/(decreases).
[2] | The range and weighted average represent the percentage of fair value derived from the unobservable inputs.
[3] | As the forward price of natural gas increases/(decreases), the fair value of purchase contracts increases/(decreases). As the forward price of natural gas increases/(decreases), the fair value of sales contracts (decreases)/increases.
[4] | As forward market prices increase/(decrease), the fair value of contracts (decreases)/increases. As volumetric assumptions for contracts in a gain position increase/(decrease), the fair value of contracts increases/(decreases). As volumetric assumptions for contracts in a loss position increase/(decrease), the fair value of the contracts (decreases)/increases.
[5] | As the forward implied spread increases/(decreases), the fair value of the contracts increases/(decreases).
[6] | The proprietary model used to calculate fair value incorporates market heat rates, correlations and volatilities. As the market implied heat rate increases/(decreases), the fair value of purchased calls increases/(decreases). As the market implied heat rate increases/(decreases), the fair value of sold calls (decreases)/increases.

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