Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Costs (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 97	$ 58	$ 95
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	57	44	53
Interest cost	109	109	100
Expected return on plan assets	(140)	(141)	(145)
Amortization of prior service cost (credit)	2	2	2
Amortization of net actuarial loss	65	45	67
Net periodic benefit cost	93	59	77
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	7	7	8
Interest cost	24	26	29
Expected return on plan assets	(22)	(24)	(20)
Amortization of prior service cost (credit)	(13)	(13)	(11)
Amortization of net actuarial loss	8	3	12
Net periodic benefit cost	$ 4	$ (1)	$ 18