Financial Statement Presentation New Accounting Pronouncement Adopted (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Electrical Distribution Revenue	$ 722	$ 813
Electrical Generation Revenue	2,018	1,768
Electric Revenue	(2,740)	(2,581)
Cost of Transmission	(601)	(703)
Electric Production Expense	(1,483)	(1,321)
Cost of Goods Sold, Electric	2,084	2,024
Gross Profit	(656)	(557)
General and Administrative Expense	(56)	(54)
Interest Expense	(281)	(287)
Investment Income, Interest	(76)	(63)
Other Assets, Current	(703)		$ (691)	$ (630)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	(94)		(106)	(130)
Service Concession Assets	0		0	1,360
Assets	32,573			33,112
Notes, Loans and Financing Receivable, Gross, Noncurrent	1,474		1,490	0
Accrued Liabilities, Current	1,182			1,232
Other Liabilities, Noncurrent	2,264			2,365
Retained Earnings (Accumulated Deficit)	(1,525)		(2,209)	(2,276)
Accumulated Other Comprehensive Income (Loss), Net of Tax	(1,808)		(1,857)	(1,876)
Cash and Cash Equivalents, at Carrying Value	(1,212)			(949)
Restricted Cash and Cash Equivalents, Current	(415)			(274)
Other Restricted Assets, Noncurrent	(541)			(565)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	2,168	2,248		1,788	$ 1,960
Stockholders' Equity Attributable to Noncontrolling Interest	2,332		2,461	2,380
Liabilities and Equity	32,573			33,112
Gain (Loss) on Extinguishment of Debt	170	(17)
Other Expenses	(9)	(24)
Other Income	(13)	(73)
Gain (Loss) on Disposition of Business	(788)	0
Foreign Currency Transaction Gain (Loss), before Tax	19	20
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(998)	(157)
Income Tax Expense (Benefit)	(231)	(67)
Income (Loss) from Equity Method Investments	(11)	(7)
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	778	97
Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, Net of Tax	1	(1)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	777	98
Net Income (Loss) Attributable to Noncontrolling Interest	(93)	(121)
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(685)	24
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	1	0
Net Income (Loss) Attributable to Parent	(684)	24
ASC 606 Impact [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Electric Revenue	(11)
Cost of Goods Sold, Electric	6
Gross Profit	5
Investment Income, Interest	(15)
Other Assets, Current	(63)		(61)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	(24)		24
Service Concession Assets	(1,337)		(1,360)
Assets	176
Notes, Loans and Financing Receivable, Gross, Noncurrent	1,474		1,490
Accrued Liabilities, Current	1
Retained Earnings (Accumulated Deficit)	76		67
Accumulated Other Comprehensive Income (Loss), Net of Tax	19		19
Stockholders' Equity Attributable to Noncontrolling Interest	80		$ 81
Liabilities and Equity	176
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(10)
Income Tax Expense (Benefit)	(1)
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	9
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	9
Net Income (Loss) Attributable to Parent	(9)
Balance Without the Adoption of ASC 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Electric Revenue	(2,751)
Cost of Goods Sold, Electric	2,090
Gross Profit	(661)
Investment Income, Interest	(61)
Other Assets, Current	(640)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	(118)
Service Concession Assets	1,337
Assets	32,397
Notes, Loans and Financing Receivable, Gross, Noncurrent	0
Accrued Liabilities, Current	1,181
Retained Earnings (Accumulated Deficit)	(1,601)
Accumulated Other Comprehensive Income (Loss), Net of Tax	(1,827)
Stockholders' Equity Attributable to Noncontrolling Interest	2,252
Liabilities and Equity	32,397
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(988)
Income Tax Expense (Benefit)	(230)
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	769
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	768
Net Income (Loss) Attributable to Parent	(675)
Accounting Standard Update 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets				23
Other Expense [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Expenses	$ (9)	$ (24)
Inc (Dec) in Cash provided by Operating Actitvities [Domain] | Accounting Standard Update 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets				5
Dec (Inc) In Net Cash Used by Financing Activities [Domain] | Accounting Standard Update 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets				$ 0