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		Sep. 30, 2016	Dec. 31, 2015
Auction Rate Securities
Fair Value Inputs [Abstract]
Asset, fair value net	[1]	$ 3	$ 6
Auction Rate Securities | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Modeled from SIFMA Index	[2]	38.00%	46.00%
Auction Rate Securities | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Modeled from SIFMA Index	[2]	38.00%	47.00%
Auction Rate Securities | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Modeled from SIFMA Index	[2]	38.00%	46.50%
Natural gas contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative, fair value, net	[3]	$ 31	$ 55
Natural gas contracts | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Proprietary model used to calculate forward prices	[2]	13.00%	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]	69.00%	50.00%
Power sales contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative, fair value, net	[4]	$ 12	$ 13
Power sales contracts | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Proprietary model used to calculate forward prices	[2]	100.00%	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]	100.00%	100.00%
FTR purchase contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative, fair value, net	[5]	$ 1	$ (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]	$ (1)	$ (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.