Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred tax liabilities
Property related	$ 1,121	$ 1,036
Pension and postretirement benefits	116	111
Total deferred tax liabilities	1,237	1,147
Deferred tax assets
Loss and credit carryforwards	301	243
Medical benefits	27	27
Insurance reserves	16	16
Pension and postretirement benefits	66	63
Capitalized energy conservation assistance costs	18	17
Other	26	23
Total deferred tax assets	454	389
Total deferred tax liability, net	$ 783	$ 758