Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Pension Plan [Member]
Defined Benefit Plan, Service Cost	[1]	$ 1	$ 1
Defined Benefit Plan, Interest Cost	[1]	24	24
Defined Benefit Plan, Expected Return on Plan Assets	[1]	(48)	(46)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	[1]	0	0
Defined Benefit Plan, Amortization of Gains (Losses)	[1]	6	6
Defined Benefit Plan, Net Periodic Benefit Cost	[1]	(17)	(15)
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Service Cost		0	0
Defined Benefit Plan, Interest Cost		0	0
Defined Benefit Plan, Expected Return on Plan Assets		(1)	(1)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		(1)	(1)
Defined Benefit Plan, Amortization of Gains (Losses)		0	0
Defined Benefit Plan, Net Periodic Benefit Cost		$ (2)	$ (2)

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