Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Costs (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 15	$ 12	$ 43	$ 33
Interest cost	28	27	82	82
Expected return on plan assets	(35)	(36)	(105)	(107)
Amortization of prior service cost (benefit)		1	1	2
Amortization of net actuarial loss	16	12	49	34
Net periodic benefit cost	24	16	70	44
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	1	5	5
Interest cost	6	7	18	20
Expected return on plan assets	(6)	(6)	(17)	(18)
Amortization of prior service cost (benefit)	(3)	$ (2)	(9)	(9)
Amortization of net actuarial loss	2		6	$ 2
Net periodic benefit cost	$ 1		$ 3