Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Costs (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 14	$ 12
Interest cost	27	27
Expected return on plan assets	(35)	(35)
Amortization of net actuarial loss	16	11
Net periodic benefit cost	22	15
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	2
Interest cost	6	7
Expected return on plan assets	(6)	(6)
Amortization of prior service cost (benefit)	(3)	(3)
Amortization of net actuarial loss	3	3
Net periodic benefit cost	$ 2	$ 3