Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 12	$ 13
Interest cost	27	25
Expected return on plan assets	(35)	(37)
Amortization of net actuarial loss	11	16
Net periodic benefit cost	15	17
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	2
Interest cost	7	8
Expected return on plan assets	(6)	(5)
Amortization of prior service cost (benefit)	(3)	(1)
Amortization of net actuarial loss	3	4
Net periodic benefit cost	$ 3	$ 8