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
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 98	$ 121	$ 99
Interest cost	201	191	184
Expected return on plan assets	(180)	(163)	(138)
Curtailment loss		2
Amortization of prior service credits			(1)
Settlement (gain) loss	1	(10)	1
Amortization of unrecognized actuarial (gain) loss	12	48	21
Net periodic benefit cost	132	189	166
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	19	52	50
Interest cost	88	110	124
Expected return on plan assets	(2)	(2)	(2)
Curtailment loss		2
Amortization of prior service credits	(31)	(3)
Amortization of unrecognized actuarial (gain) loss	(47)	3	(3)
Net periodic benefit cost	$ 27	$ 162	$ 169