Regulatory Matters - Regulatory Assets (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Regulatory assets		$ 150	$ 130
Regulatory assets, noncurrent		4,307	4,155
Total regulatory assets		4,457	4,285
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided		2,800	2,300
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Total regulatory assets	[1]	$ 1,754	1,577
Regulatory asset amortization period years	[1]	27 years
Employee benefit plans [Member]
Regulatory Assets [Line Items]
Total regulatory assets	[2]	$ 816	778
Regulatory asset amortization period years	[2]	17 years
Asset disposition costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 281	307
Regulatory asset amortization period years		Various
Deferred net power costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 38	140
Regulatory asset amortization period years		1 year
Asset Retirement Obligation Costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 301	281
Regulatory asset amortization period years		12 years
Unrealized loss on regulated derivative contracts [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 154	250
Regulatory asset amortization period years		5 years
Abandoned Projects [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 159	136
Regulatory asset amortization period years		3 years
Unamortized contract values [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 98	110
Regulatory asset amortization period years		7 years
Regulatory assets other [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 856	$ 706
Regulatory asset amortization period years		Various

[1] Amounts primarily represent income tax benefits related to state accelerated tax depreciation and certain property-related basis differences that were previously flowed through to customers and will be included in regulated rates when the temporary differences reverse.

[2] Represents amounts not yet recognized as a component of net periodic benefit cost that are expected to be included in regulated rates when recognized.