Income Taxes - Components of Deferred Tax Liabilities and Deferred Tax Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred Tax Liabilities
Property, plant and equipment and investments	$ (847)	$ (1,318)
Other	(45)	(45)
Total Deferred Tax Liabilities	(892)	(1,363)
Deferred Tax Assets
Net operating loss carryforwards	5,037	4,733
Tax credit carryforwards	135	66
Property, plant and equipment and investments	55	206
Accrued compensation, deferred credits and other liabilities	196	179
Asset retirement obligations	252	261
Other	325	317
Total Deferred Tax Assets	6,000	5,762
Valuation allowances	(5,391)	(4,734)
Total deferred tax assets, net of valuation allowances	609	1,028
Net Deferred Tax Assets (Liabilities)	(283)	(335)
Valuation allowance increase (decrease) amount	$ 657	$ (143)	$ (322)