Regulatory Matters - Regulatory Assets (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Regulatory assets		$ 130	$ 253
Regulatory assets, noncurrent		4,155	4,000
Total regulatory assets		4,285	4,253
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided		2,300	2,600
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Total regulatory assets	[1]	$ 1,577	1,468
Regulatory asset amortization period years	[1]	26 years
Employee benefit plans [Member]
Regulatory Assets [Line Items]
Total regulatory assets	[2]	$ 778	747
Regulatory asset amortization period years	[2]	9 years
Asset disposition costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets	[3]	$ 307	329
Regulatory asset amortization period years	[3]	Various
Deferred net power costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 140	277
Regulatory asset amortization period years		1 year
Asset Retirement Obligation Costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 281	239
Regulatory asset amortization period years		8 years
Unrealized loss on regulated derivative contracts [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 250	223
Regulatory asset amortization period years		5 years
Abandoned Projects [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 136	159
Regulatory asset amortization period years		5 years
Unamortized contract values [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 110	123
Regulatory asset amortization period years		8 years
Regulatory assets other [Member]
Regulatory Assets [Line Items]
Total regulatory assets		$ 706	$ 688
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.

[3] Includes amounts established as a result of the Utah mine disposition discussed below for the net property, plant and equipment not considered probable of disallowance and for the portion of losses associated with the assets held for sale, UMWA 1974 Pension Plan withdrawal and closure costs incurred to date considered probable of recovery.