Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 335	$ 391	$ 288
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	166	194	186
Interest cost	630	602	639
Expected return on plan assets	(782)	(788)	(808)
Recognized net actuarial loss	391	489	384
Amortization of prior service cost (benefit)	18	19	19
Settlements	26	9
Curtailments		6
Special termination benefits		77
Net periodic benefit cost	449	608	420
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	15	17	14
Interest cost	114	114	131
Recognized net actuarial loss	13	35	25
Amortization of prior service cost (benefit)	(25)	(18)	(16)
Net periodic benefit cost	$ 117	$ 148	$ 154