Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Pension Plan [Member]
Defined Benefit Plan, Service Cost	[1]	$ 5	[2]	$ 5	[2]	$ 5
Defined Benefit Plan, Interest Cost	[1]	97	[2]	116	[2]	124
Defined Benefit Plan, Expected Return on Plan Assets	[1]	(186)		(206)		(200)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	[1]	0		0		0
Defined Benefit Plan, Amortization of Gains (Losses)	[1]	25		21		16
Defined Benefit Plan, Net Periodic Benefit Cost	[1]	$ (59)		$ (64)		$ (55)
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	[1]	4.44%		4.09%		4.98%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	[1]	6.68%		6.91%		7.17%
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Service Cost		$ 0		$ 0		$ 0
Defined Benefit Plan, Interest Cost		2		2		3
Defined Benefit Plan, Expected Return on Plan Assets		(5)		(5)		(5)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		(6)		(6)		(6)
Defined Benefit Plan, Amortization of Gains (Losses)		0		0		0
Defined Benefit Plan, Net Periodic Benefit Cost		$ (9)		$ (9)		$ (8)
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate		3.95%		3.60%		4.15%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets		3.13%		3.50%		3.68%

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

[2] Employer contributions represent the benefits that were paid to nonqualified plan participants. Unfunded nonqualified supplemental pension plans are not funded through plan assets.