Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Plan [Member]
Defined Benefit Plan, Service Cost	[1]	$ 1	$ 1	$ 4	$ 4
Defined Benefit Plan, Interest Cost		23	24	68	71
Defined Benefit Plan, Expected Return (Loss) on Plan Assets		(47)	(49)	(140)	(146)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		0	0	0	0
Defined Benefit Plan, Amortization of Gain (Loss)		6	6	17	18
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		(17)	(18)	(51)	(53)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Service Cost		0	0	0	0
Defined Benefit Plan, Interest Cost		0	0	1	1
Defined Benefit Plan, Expected Return (Loss) on Plan Assets		(1)	(1)	(4)	(4)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		(2)	(1)	(5)	(4)
Defined Benefit Plan, Amortization of Gain (Loss)		0	0	0	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		$ (3)	$ (2)	$ (8)	$ (7)

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