Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 40	$ 44
Interest cost	122	144
Expected return on plan assets	(185)	(188)
Recognized net actuarial loss	102	118
Amortization of prior service cost (benefit)	4	4
Settlements		1
Special termination benefits	1	2
Net periodic benefit cost	84	125
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	3
Interest cost	18	23
Recognized net actuarial loss	6	4
Amortization of prior service cost (benefit)	(6)	(9)
Net periodic benefit cost	$ 21	$ 21