Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Regulatory Assets and Liabilities [Line Items]
Regulatory Charges In Operating Expense Resulting From Tax Reform	$ (17)	$ 674	$ 0
Regulatory Charges Resulting From Tax Reform	(15)	776	$ 0
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets, Current	103	102
Regulatory Assets, Noncurrent	495	376
Total regulatory assets	598	478
Regulatory Liabilities, Current	5	18
Regulatory Liabilities, Noncurrent	1,321	1,250
Total regulatory liabilities	$ 1,326	$ 1,268
Interest Capitalized [Abstract]
Minimum period of construction for capitalization of interest	3 months
Minimum total project cost for capitalization of interest	$ 1
Retirement Benefits, Description [Abstract]
Threshold For Amortization Of Unrecognized Actuarial Gains Losses	10.00%
Pension Benefits [Member]
Retirement Benefits, Description [Abstract]
Approximate Amortization Period Of Net Actuarial Gain Loss	13 years
Amortization Period Of Difference Between Expected And Actual Return On Plan Assets	5 years
Other Postretirement Benefits [Member]
Retirement Benefits, Description [Abstract]
Approximate Amortization Period Of Net Actuarial Gain Loss	7 years
Maximum [Member]
Regulatory Assets and Liabilities [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent		35.00%
Maximum [Member] | Pension Benefits [Member]
Retirement Benefits, Description [Abstract]
Threshold For Market Related Value	110.00%
Minimum [Member]
Regulatory Assets and Liabilities [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent		21.00%
Minimum [Member] | Pension Benefits [Member]
Retirement Benefits, Description [Abstract]
Threshold For Market Related Value	90.00%
Nonoperating Income (Expense) [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Charges Resulting From Tax Reform		$ 11
Other Nonoperating Income (Expense) [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Charges Resulting From Tax Reform		$ 102
Accounting Standards Update 2014-09 [Member]
Regulatory Assets and Liabilities [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets				$ (121)
Retained Earnings [Member] | Accounting Standards Update 2018-02 [Member]
Regulatory Assets and Liabilities [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets				61
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accounting Standards Update 2018-02 [Member]
Regulatory Assets and Liabilities [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets				$ (61)
Regulatory Charges In Operating Expenses Resulting From Tax Reform [Member]
Regulatory Assets and Liabilities Disclosure [Abstract]
Total regulatory liabilities	$ 657