Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 12	$ 16	$ 22
Interest cost	127	149	136
Expected return on plan assets	(193)	(209)	(198)
Amortization of unrecognized net loss	42	30	54
Amortization of unrecognized prior service benefit	(1)	(1)
Settlement/Curtailment	3	86	5
Net periodic benefit cost	(10)	71	19
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	1
Interest cost	3	4	4
Expected return on plan assets	(5)	(4)	(5)
Amortization of unrecognized net loss	1	1	1
Amortization of unrecognized prior service benefit	(10)	(18)	(25)
Settlement/Curtailment		(86)
Net periodic benefit cost	$ (10)	$ (102)	$ (24)