Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Costs (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 14	$ 9	$ 28	$ 21
Interest cost	27	28	54	55
Expected return on plan assets	(35)	(36)	(70)	(71)
Amortization of prior service cost (benefit)	1	1	1	1
Amortization of net actuarial loss	17	11	33	22
Net periodic benefit cost	24	13	46	28
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	2	3	4
Interest cost	6	6	12	13
Expected return on plan assets	(5)	(6)	(11)	(12)
Amortization of prior service cost (benefit)	(3)	(4)	(6)	(7)
Amortization of net actuarial loss	$ 1	(1)	4	$ 2
Net periodic benefit cost		$ (3)	$ 2