Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred tax liabilities
Property related	$ 924	$ 1,549
Pension and postretirement benefits	57	105
Pension	43	69
Total deferred tax liabilities	1,024	1,723
Deferred tax assets
Loss and credit carryforwards	91	91
Medical benefits	24	47
Insurance reserves	(5)	27
Pension and postretirement benefits	57	105
Capitalized energy conservation assistance costs	13	23
Other	19	23
Total deferred tax assets	199	316
Total deferred tax liability, net	$ 825	$ 1,407