Defined Benefit Pension and Other Postretirement Plans - Components of Net Periodic Benefit Costs (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 29	$ 28	$ 57	$ 51
Interest cost	18	18	37	36
Expected return on plan assets	(24)	(24)	(49)	(50)
Amortization – prior service credit	(12)	(11)	(23)	(23)
Amortization – actuarial loss	8	13	18	26
Amortization - settlement loss	2	1	3	2
Net periodic benefit cost	21	25	43	42
Other Benefits
Components of net periodic benefit cost:
Service cost	8	7	16	15
Interest cost	8	8	17	16
Expected return on plan assets	0	0	0	0
Amortization – prior service credit	0	(1)	(1)	(2)
Amortization – actuarial loss	0	1	1	4
Amortization - settlement loss	0	0	0	0
Net periodic benefit cost	$ 16	$ 15	$ 33	$ 33