Amounts Recognized in AOCI (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax		$ 6	$ 6	$ 6
Deferred losses related to NCF Retirement Plan settlement	[1]	60	0	0
Pension Plan [Member]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, Prior Service Cost (Credit), before Tax		0	0
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Gain (Loss), before Tax		978	1,001
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax		978	1,001
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax		59
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax		0
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Reclassification Adjustment, before Tax		(22)
Deferred losses related to NCF Retirement Plan settlement		(60)	0	0
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, before Tax		(23)
Defined Benefit Plans Recognized in Periodic Benefit Cost and Accumulated Comprehensive Income		(31)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, Prior Service Cost (Credit), before Tax		(52)	(58)
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Gain (Loss), before Tax		(17)	(22)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax		(69)	(80)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax		5
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax		6
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Reclassification Adjustment, before Tax		0
Deferred losses related to NCF Retirement Plan settlement		0	$ 0	$ 0
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, before Tax		(11)
Defined Benefit Plans Recognized in Periodic Benefit Cost and Accumulated Comprehensive Income		$ 1

[1] Related to the Company's NCF Retirement Plan settlement in the fourth quarter of 2018. See Note 17, "Employee Benefit Plans," for additional information.