Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Pension Plan [Member]
Defined Benefit Plan, Service Cost	[1]	$ 1	$ 1
Defined Benefit Plan, Interest Cost		23	24
Defined Benefit Plan, Expected Return (Loss) on Plan Assets		47	48
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		0	0
Defined Benefit Plan, Amortization of Gain (Loss)		(6)	(6)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	[1]	(17)	(17)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Service Cost		0	0
Defined Benefit Plan, Interest Cost		0	0
Defined Benefit Plan, Expected Return (Loss) on Plan Assets		1	1
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		(2)	(1)
Defined Benefit Plan, Amortization of Gain (Loss)		0	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		$ (3)	$ (2)

[1] Administrative fees are recognized in service cost for each of the periods presented.