Summary of Significant Accounting Policies (Narrative) (Detail)		12 Months Ended
	Jan. 01, 2019 USD ($)	Dec. 31, 2019 USD ($) MW		Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Significant Accounting Policies
Carrying value of property		$ 69,000,000		$ 72,000,000
Public utilities, allowance for funds used during construction, rate		4.30%		7.00%	3.90%
Utilities operating expense, maintenance and operations, affiliate		$ 244,000,000		$ 182,000,000	$ 180,000,000
Turbine maintenance expense		10,000,000		16,000,000
Amount accrued annually for nuclear fuel outages		17,000,000
Nuclear refueling outage cost		2,000,000		29,000,000
Decommissioning liability, noncurrent		646,000,000
Payments to acquire investments to be held in decommissioning trust fund		3,000,000
Restricted cash and equivalent balances		0		0	$ 0
Cash held in escrow				115,000,000
Unbilled revenues		$ 114,000,000		$ 129,000,000
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%		21.00%	35.00%
Unrecognized tax benefits		$ 132,000,000		$ 106,000,000	$ 98,000,000	$ 350,000,000
Interest expense		18,000,000		8,000,000
Interest income		2,000,000		2,000,000
Penalty expenses		7,000,000
Income taxes related to affiliated receivable		21,000,000
Margin liabilities with cash collateral		0		0
Margin assets with cash collateral		$ 19,000,000		$ 11,000,000
Option to extend, existence, operating lease		true
Operating lease, right of use asset	$ 19,000,000	$ 23,000,000	[1]
Operating lease liability	19,000,000	$ 23,000,000
Reclassification From AOCI to retained earnings	$ 1,000,000
Minimum [Member]
Significant Accounting Policies
Lease renewal term		1 year
Maximum [Member]
Significant Accounting Policies
Interest expense					1,000,000
Interest income					$ 1,000,000
Lease renewal term		70 years
Original term of leases		1 year
Accounts Receivable | Credit Concentration Risk
Significant Accounting Policies
Concentration risk percentage		10.00%
Turbine [Member]
Significant Accounting Policies
Utilities operating expense, maintenance and operations, affiliate		$ 18,000,000
Genco
Significant Accounting Policies
Power Generation Capacity Megawatts | MW		605
Carrying value of property		$ 508,000,000

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