Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Pension Plan [Member]
Defined Benefit Plan, Service Cost	[1]	$ 5	[2]	$ 5	[2]	$ 5
Defined Benefit Plan, Interest Cost	[1]	116	[2]	124	[2]	113
Defined Benefit Plan, Expected Return on Plan Assets	[1]	(206)		(200)		(192)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		0		0		0
Defined Benefit Plan, Amortization of Gains (Losses)	[1]	21		16		26
Defined Benefit Plan, Net Periodic Benefit Cost	[1]	$ (64)		$ (55)		$ (48)
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	[1]	4.09%		4.98%		4.08%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	[1]	6.91%		7.17%		7.00%
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Service Cost		$ 0		$ 0		$ 0
Defined Benefit Plan, Interest Cost		2		3		6
Defined Benefit Plan, Expected Return on Plan Assets		(5)		(5)		(6)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		(6)		(6)		0
Defined Benefit Plan, Amortization of Gains (Losses)		0		0		0
Defined Benefit Plan, Net Periodic Benefit Cost		$ (9)		$ (8)		$ 0
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate		3.60%		4.15%		3.45%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	[3]	3.50%		3.68%		3.49%

[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.

[3] The weighted average shown is determined on an after-tax basis.