Commitments and Guarantees Unrecorded Unconditional Purchase Obligations (Details) $ in Millions	Dec. 31, 2016 USD ($)
Capital Additions [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 256
Unrecorded Unconditional Purchase Obligation, Due within One Year	176
Unrecorded Unconditional Purchase Obligation, Due within Two Years	8
Unrecorded Unconditional Purchase Obligation, Due within Three Years	2
Unrecorded Unconditional Purchase Obligation, Due within Four Years	9
Unrecorded Unconditional Purchase Obligation, Due within Five Years	1
Unrecorded Unconditional Purchase Obligation, Due after Five Years	60
Long-term Contract for Purchase of Electric Power [Domain]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation	2,804
Unrecorded Unconditional Purchase Obligation, Due within One Year	221
Unrecorded Unconditional Purchase Obligation, Due within Two Years	157
Unrecorded Unconditional Purchase Obligation, Due within Three Years	181
Unrecorded Unconditional Purchase Obligation, Due within Four Years	256
Unrecorded Unconditional Purchase Obligation, Due within Five Years	239
Unrecorded Unconditional Purchase Obligation, Due after Five Years	1,750
Electric Transmission [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation	38
Unrecorded Unconditional Purchase Obligation, Due within One Year	7
Unrecorded Unconditional Purchase Obligation, Due within Two Years	6
Unrecorded Unconditional Purchase Obligation, Due within Three Years	5
Unrecorded Unconditional Purchase Obligation, Due within Four Years	4
Unrecorded Unconditional Purchase Obligation, Due within Five Years	4
Unrecorded Unconditional Purchase Obligation, Due after Five Years	12
Public Utility Districts [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation	21
Unrecorded Unconditional Purchase Obligation, Due within One Year	4
Unrecorded Unconditional Purchase Obligation, Due within Two Years	4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	1
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	1
Unrecorded Unconditional Purchase Obligation, Due after Five Years	11
Natural gas [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation	333
Unrecorded Unconditional Purchase Obligation, Due within One Year	53
Unrecorded Unconditional Purchase Obligation, Due within Two Years	39
Unrecorded Unconditional Purchase Obligation, Due within Three Years	32
Unrecorded Unconditional Purchase Obligation, Due within Four Years	27
Unrecorded Unconditional Purchase Obligation, Due within Five Years	24
Unrecorded Unconditional Purchase Obligation, Due after Five Years	158
Coal and transportationSupply Agreements [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation	31
Unrecorded Unconditional Purchase Obligation, Due within One Year	17
Unrecorded Unconditional Purchase Obligation, Due within Two Years	9
Unrecorded Unconditional Purchase Obligation, Due within Three Years	5
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
Commitments [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation	3,483
Unrecorded Unconditional Purchase Obligation, Due within One Year	478
Unrecorded Unconditional Purchase Obligation, Due within Two Years	223
Unrecorded Unconditional Purchase Obligation, Due within Three Years	226
Unrecorded Unconditional Purchase Obligation, Due within Four Years	296
Unrecorded Unconditional Purchase Obligation, Due within Five Years	269
Unrecorded Unconditional Purchase Obligation, Due after Five Years	$ 1,991