Pension and Other Benefit Plans - Changes in Accumulated Postretirement Benefit Obligation, Plan Assets and Funded Status (Detail) - Postretirement medical plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Change in Projected Benefit Obligations
Projected benefit obligation at beginning of year	$ 1,221	$ 1,247
Service cost	42	43
Interest cost	48	60
Contribution by plan participants	7	6
Actuarial (gains) losses	(168)	210
Benefits paid	(47)	(46)
Plan amendments		(299)
Projected benefit obligation at end of year	1,103	1,221
Change in Plan Assets
Plan assets at fair value at beginning of year	854	731
Company contributions	66	65
Contribution by plan participants	7	6
Benefits paid	(47)	(46)
Actual return on plan assets	4	98
Plan assets at fair value at end of year	884	854
Unfunded Liability	(219)	(367)
Amounts Recognized in Accumulated Other Comprehensive Loss
Actuarial (gains) losses	106	242
Prior service cost	(275)	(307)
Total amount recognized in accumulated other comprehensive income	$ (169)	$ (65)