Summary of Significant Accounting Policies (Narrative) (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($) MW		May 31, 2019	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)
Carrying value of property	$ 71				$ 72
Option to extend, existence, operating lease	true
Operating lease, right of use asset	$ 20	[1]		$ 19
Operating lease liability	$ 18			$ 19
Minimum [Member]
Power Generation Capacity Megawatts | MW	25
Lease renewal term	1 year
Maximum [Member]
Lease renewal term	70 years
Genco
Power Generation Capacity Megawatts | MW	605
Carrying value of property	$ 497
Genco | 5.49% Senior Secured Note Due 2024
Debt instrument, interest rate			5.49%

[1] Included in other deferred debits and other assets in the Consolidated Balance Sheets.