Summary of Significant Accounting Policies - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	$ 55	$ 66
Non-current regulatory assets	212	245
Total regulatory assets(a)	267	311
Current regulatory liabilities	26	29
Non-current regulatory liabilities	189	206
Total regulatory liabilities(b)	215	$ 235
Regulatory assets recoverable without earning a return	$ 84
Regulatory assets, weighted average remaining recovery period	26 years
Remaining Amounts of Regulatory Liabilities Subject to Crediting Period	$ 131
Remaining Recovery Period of Regulatory Liabilities Subject to Defined Crediting Period	18 years
Remaining Amounts of Regulatory Liabilities Not Subject to Defined Crediting Period	$ 58
Loss on Disposal of Assets [Member]
Regulatory Assets and Liabilities [Line Items]
Total regulatory assets(a)	144
Income Tax Gross Up on AFUDC Equity [Member]
Regulatory Assets and Liabilities [Line Items]
Total regulatory assets(a)	51
Other Regulatory Assets (Liabilities) [Member]
Regulatory Assets and Liabilities [Line Items]
Total regulatory assets(a)	$ 72