Commitments and Contingencies - Commitments Table (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Contractual Obligation [Line Items]
Total commitments, Due in Next Twelve Months	$ 762
Total commitments, Due in Second Year	571
Total commitments, Due in Third Year	487
Total commitments, Due in Fourth Year	413
Total commitments, Due in Fifth Year	423
Total commitments, Due after Thereafter	5,154
Total commitments	7,810us-gaap_ContractualObligation
Fuel, capacity and transmission contract commitments [Member]
Contractual Obligation [Line Items]
Purchase Obligation, Due in Next Twelve Months	607
Purchase Obligation, Due in Second Year	447
Purchase Obligation, Due in Third Year	363
Purchase Obligation, Due in Fourth Year	289
Purchase Obligation, Due in Fifth Year	290
Purchase Obligation, Due Thereafter	3,058
Purchase Obligation	5,054
Fuel, capacity and transmission contract commitments, Not commercially operable [Member]
Contractual Obligation [Line Items]
Purchase Obligation, Due in Next Twelve Months	65
Purchase Obligation, Due in Second Year	79
Purchase Obligation, Due in Third Year	80
Purchase Obligation, Due in Fourth Year	81
Purchase Obligation, Due in Fifth Year	88
Purchase Obligation, Due Thereafter	1,883
Purchase Obligation	2,276
Capital Addition Purchase Commitments [Member]
Contractual Obligation [Line Items]
Purchase Obligation, Due in Next Twelve Months	42
Purchase Obligation, Due in Second Year	0
Purchase Obligation, Due in Third Year	0
Purchase Obligation, Due in Fourth Year	0
Purchase Obligation, Due in Fifth Year	0
Purchase Obligation, Due Thereafter	0
Purchase Obligation	42
Operating Leases, Easements, Maintenance and Service [Member]
Contractual Obligation [Line Items]
Purchase Obligation, Due in Next Twelve Months	9
Purchase Obligation, Due in Second Year	7
Purchase Obligation, Due in Third Year	6
Purchase Obligation, Due in Fourth Year	7
Purchase Obligation, Due in Fifth Year	7
Purchase Obligation, Due Thereafter	52
Purchase Obligation	88
Maintenance, service and other contracts [Member]
Contractual Obligation [Line Items]
Purchase Obligation, Due in Next Twelve Months	39
Purchase Obligation, Due in Second Year	38
Purchase Obligation, Due in Third Year	38
Purchase Obligation, Due in Fourth Year	36
Purchase Obligation, Due in Fifth Year	38
Purchase Obligation, Due Thereafter	161
Purchase Obligation	$ 350

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Amount of contractual obligation, including but not limited to, long-term debt, capital lease obligations, operating lease obligations, purchase obligations, and other commitments.

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Amount of contractual obligation maturing in the fifth fiscal year following the latest fiscal year.

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Amount of contractual obligation maturing in the fourth fiscal year following the latest fiscal year.

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Minimum amount of purchase arrangement in which the entity has agreed to expend funds to procure goods or services from a supplier.

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Minimum amount of purchase arrangement maturing after the fifth fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

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Minimum amount of purchase arrangement maturing in the fifth fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

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Minimum amount of purchase arrangement maturing in the fourth fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

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Minimum amount of purchase arrangement maturing in the next fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

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Minimum amount of purchase arrangement maturing in the third fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

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