Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Costs (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 58	$ 95	$ 110
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	44	53	35
Interest cost	109	100	107
Expected return on plan assets	(141)	(145)	(132)
Amortization of prior service cost (credit)	2	2	1
Amortization of net actuarial loss	45	67	64
Net periodic benefit cost	59	77	75
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	7	8	7
Interest cost	26	29	35
Expected return on plan assets	(24)	(20)	(18)
Amortization of prior service cost (credit)	(13)	(11)	(4)
Amortization of net actuarial loss	3	12	14
Termination benefits			1
Net periodic benefit cost	$ (1)	$ 18	$ 35