Commitments and Guarantees Unrecorded Unconditional Purchase Obligations (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Capital Additions [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 185
Unrecorded Unconditional Purchase Obligation, Due within One Year	81
Unrecorded Unconditional Purchase Obligation, Due within Two Years	10
Unrecorded Unconditional Purchase Obligation, Due within Three Years	11
Unrecorded Unconditional Purchase Obligation, Due within Four Years	9
Unrecorded Unconditional Purchase Obligation, Due within Five Years	2
Unrecorded Unconditional Purchase Obligation, Due after Five Years	72
Long-term Contract for Purchase of Electric Power [Domain]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation	859
Unrecorded Unconditional Purchase Obligation, Due within One Year	154
Unrecorded Unconditional Purchase Obligation, Due within Two Years	83
Unrecorded Unconditional Purchase Obligation, Due within Three Years	82
Unrecorded Unconditional Purchase Obligation, Due within Four Years	64
Unrecorded Unconditional Purchase Obligation, Due within Five Years	36
Unrecorded Unconditional Purchase Obligation, Due after Five Years	440
Electric Transmission [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation	81
Unrecorded Unconditional Purchase Obligation, Due within One Year	21
Unrecorded Unconditional Purchase Obligation, Due within Two Years	21
Unrecorded Unconditional Purchase Obligation, Due within Three Years	20
Unrecorded Unconditional Purchase Obligation, Due within Four Years	19
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Public Utility Districts [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation	61
Unrecorded Unconditional Purchase Obligation, Due within One Year	8
Unrecorded Unconditional Purchase Obligation, Due within Two Years	8
Unrecorded Unconditional Purchase Obligation, Due within Three Years	8
Unrecorded Unconditional Purchase Obligation, Due within Four Years	7
Unrecorded Unconditional Purchase Obligation, Due within Five Years	5
Unrecorded Unconditional Purchase Obligation, Due after Five Years	25
Natural gas [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation	130
Unrecorded Unconditional Purchase Obligation, Due within One Year	55
Unrecorded Unconditional Purchase Obligation, Due within Two Years	26
Unrecorded Unconditional Purchase Obligation, Due within Three Years	21
Unrecorded Unconditional Purchase Obligation, Due within Four Years	12
Unrecorded Unconditional Purchase Obligation, Due within Five Years	10
Unrecorded Unconditional Purchase Obligation, Due after Five Years	6
Coal and transportationSupply Agreements [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation	31
Unrecorded Unconditional Purchase Obligation, Due within One Year	22
Unrecorded Unconditional Purchase Obligation, Due within Two Years	9
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Operatomg leases [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation	234
Unrecorded Unconditional Purchase Obligation, Due within One Year	9
Unrecorded Unconditional Purchase Obligation, Due within Two Years	9
Unrecorded Unconditional Purchase Obligation, Due within Three Years	9
Unrecorded Unconditional Purchase Obligation, Due within Four Years	10
Unrecorded Unconditional Purchase Obligation, Due within Five Years	11
Unrecorded Unconditional Purchase Obligation, Due after Five Years	186
Commitments [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation	1,581
Unrecorded Unconditional Purchase Obligation, Due within One Year	350
Unrecorded Unconditional Purchase Obligation, Due within Two Years	166
Unrecorded Unconditional Purchase Obligation, Due within Three Years	151
Unrecorded Unconditional Purchase Obligation, Due within Four Years	121
Unrecorded Unconditional Purchase Obligation, Due within Five Years	64
Unrecorded Unconditional Purchase Obligation, Due after Five Years	$ 729