Income Taxes Income Taxes - Tax Reform (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of Income Tax Expense (Benefit) [Line Items]
Federal statutory income tax rate			35.00%	35.00%	35.00%
Deferred Tax Liabilities, Net			$ 8,242	$ 13,879
Regulatory Liabilities			7,511	3,120
Deferred Income Tax Expense (Benefit)			26	1,098	$ 1,274
Net income (loss) attributable to parent			2,870	2,542	2,370
Income tax (benefit) expense			(554)	403	450
Equity income (loss)			(151)	123	$ 115
Deferred Income Tax Charge [Member]
Components of Income Tax Expense (Benefit) [Line Items]
Regulatory Liabilities	[1]		$ 4,143	$ 25
Tax Cuts and Jobs Act of 2017 [Member]
Components of Income Tax Expense (Benefit) [Line Items]
Federal statutory income tax rate			21.00%
Deferred Tax Liabilities, Net	[2]		$ 7,115
Tax Cuts and Jobs Act of 2017 [Member] | Deferred Income Tax Charge [Member]
Components of Income Tax Expense (Benefit) [Line Items]
Regulatory Liabilities	[2]		5,950
Deferred Income Tax Expense (Benefit)	[2]	$ 1,150
Net income (loss) attributable to parent	[2]		$ 516
Income tax (benefit) expense	[2]	1,150
Foreign Earnings Repatriated		419
Equity income (loss)		$ 228

[1] (1)Amounts primarily represent income tax liabilities related to the federal tax rate change from 35% to 21% that are probable to be passed on to customers, offset by income tax benefits related to certain property-related basis differences and other various differences that were previously passed on to customers and will be included in regulated rates when the temporary differences reverse. See Note 11 for further discussion of 2017 Tax Reform impacts.

[2] (1)Amounts primarily represent income tax liabilities related to the federal tax rate change from 35% to 21% that are probable to be passed on to customers, offset by income tax benefits related to accelerated tax depreciation and certain property-related basis differences that were previously passed on to customers and will be included in regulated rates when the temporary differences reverse. See Note 10 for further discussion of 2017 Tax Reform impacts.