Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 423	$ 335	$ 391
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	175	166	194
Interest cost	577	630	602
Expected return on plan assets	(753)	(782)	(788)
Recognized net actuarial loss	468	391	489
Amortization of prior service cost (benefit)	16	18	19
Settlements	16	26	9
Curtailments	9		6
Special termination benefits	16		77
Net periodic benefit cost	524	449	608
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	14	15	17
Interest cost	92	114	114
Recognized net actuarial loss	17	13	35
Amortization of prior service cost (benefit)	(37)	(25)	(18)
Curtailments	(4)
Net periodic benefit cost	$ 82	$ 117	$ 148