Commitments, Guarantees and Contingencies - Unrecorded Contractual Obligations (Details) $ in Millions	Dec. 31, 2015 USD ($)
Unrecorded contractual obligations and commitments:
Unrecorded contractual obligations and commitments	$ 7,690
Unrecorded contractual obligations and commitments, 2016	1,094
Unrecorded contractual obligations and commitments, 2017	730
Unrecorded contractual obligations and commitments, 2018	573
Unrecorded contractual obligations and commitments, 2019	539
Unrecorded contractual obligations and commitments, 2020	466
Unrecorded contractual obligations and commitments, 2021 & thereafter	4,288
Pipeline Charges Storage Capacity And Gas Supply [Member]
Unrecorded contractual obligations and commitments:
Unrecorded contractual obligations and commitments	5,007
Unrecorded contractual obligations and commitments, 2016	795
Unrecorded contractual obligations and commitments, 2017	536
Unrecorded contractual obligations and commitments, 2018	392
Unrecorded contractual obligations and commitments, 2019	370
Unrecorded contractual obligations and commitments, 2020	318
Unrecorded contractual obligations and commitments, 2021 & thereafter	2,596
Interest Charges [Member]
Unrecorded contractual obligations and commitments:
Unrecorded contractual obligations and commitments	2,418
Unrecorded contractual obligations and commitments, 2016	181
Unrecorded contractual obligations and commitments, 2017	158
Unrecorded contractual obligations and commitments, 2018	156
Unrecorded contractual obligations and commitments, 2019	151
Unrecorded contractual obligations and commitments, 2020	133
Unrecorded contractual obligations and commitments, 2021 & thereafter	1,639
Operating Leases [Member]
Unrecorded contractual obligations and commitments:
Unrecorded contractual obligations and commitments	159
Unrecorded contractual obligations and commitments, 2016	31
Unrecorded contractual obligations and commitments, 2017	26
Unrecorded contractual obligations and commitments, 2018	18
Unrecorded contractual obligations and commitments, 2019	16
Unrecorded contractual obligations and commitments, 2020	15
Unrecorded contractual obligations and commitments, 2021 & thereafter	53
Asset Management Agreements [Member]
Unrecorded contractual obligations and commitments:
Unrecorded contractual obligations and commitments	28
Unrecorded contractual obligations and commitments, 2016	11
Unrecorded contractual obligations and commitments, 2017	9
Unrecorded contractual obligations and commitments, 2018	6
Unrecorded contractual obligations and commitments, 2019	2
Unrecorded contractual obligations and commitments, 2020	0
Unrecorded contractual obligations and commitments, 2021 & thereafter	0
Standby Letters Of Credit Performance And Surety Bonds [Member]
Unrecorded contractual obligations and commitments:
Unrecorded contractual obligations and commitments	73
Unrecorded contractual obligations and commitments, 2016	73
Unrecorded contractual obligations and commitments, 2017	0
Unrecorded contractual obligations and commitments, 2018	0
Unrecorded contractual obligations and commitments, 2019	0
Unrecorded contractual obligations and commitments, 2020	0
Unrecorded contractual obligations and commitments, 2021 & thereafter	0
Other [Member]
Unrecorded contractual obligations and commitments:
Unrecorded contractual obligations and commitments	5
Unrecorded contractual obligations and commitments, 2016	3
Unrecorded contractual obligations and commitments, 2017	1
Unrecorded contractual obligations and commitments, 2018	1
Unrecorded contractual obligations and commitments, 2019	0
Unrecorded contractual obligations and commitments, 2020	0
Unrecorded contractual obligations and commitments, 2021 & thereafter	$ 0