Pension Plans, Other Postretirement Benefits, and Defined-Contribution Plans - Components of Net Periodic Benefit Cost and Amounts Recognized in Other Comprehensive Income (Expense) Table (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Amounts recognized in other comprehensive income (expense)
Net actuarial gain (loss)	$ 49	$ (405)	$ 416
Amortization of net actuarial (gain) loss	63	27	132
Net prior service (cost) credit	89	0	0
Amortization of net prior service cost (credit)	(4)	0	1
Pension Plans, Defined Benefit [Member]
Components of net periodic benefit cost
Service cost	118	99	85
Interest cost	101	99	78
Expected return on plan assets	(109)	(106)	(91)
Amortization of net actuarial loss (gain)	52	34	118
Amortization of net prior service cost (credit)	0	0	0
Settlement loss	11	0	14
Net periodic benefit cost	173	126	204
Amounts recognized in other comprehensive income (expense)
Net actuarial gain (loss)	22	(333)	342
Amortization of net actuarial (gain) loss	52	34	118
Net prior service (cost) credit	0	0	0
Amortization of net prior service cost (credit)	0	0	0
Settlement loss	11	0	14
Total amounts recognized in other comprehensive income (expense)	85	(299)	474
Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of net periodic benefit cost
Service cost	9	7	9
Interest cost	15	15	14
Expected return on plan assets	0	0	0
Amortization of net actuarial loss (gain)	0	(7)	0
Amortization of net prior service cost (credit)	(4)	0	1
Settlement loss	0	0	0
Net periodic benefit cost	20	15	24
Amounts recognized in other comprehensive income (expense)
Net actuarial gain (loss)	27	(72)	74
Amortization of net actuarial (gain) loss	0	(7)	0
Net prior service (cost) credit	89	0	0
Amortization of net prior service cost (credit)	(4)	0	1
Settlement loss	0	0	0
Total amounts recognized in other comprehensive income (expense)	$ 112	$ (79)	$ 75