Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Deferred tax liabilities
Property related	$ 969	$ 919
Pension and postretirement benefits	105	100
Total deferred tax liabilities	1,074	1,019
Deferred tax assets
Loss and credit carryforwards	146	91
Medical benefits	24	24
Insurance reserves	17	(5)
Pension and postretirement benefits	63	57
Capitalized energy conservation assistance costs	16	13
Other	9	14
Total deferred tax assets	275	194
Total deferred tax liability, net	$ 799	$ 825