Benefit Plans - Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 1	$ 1
Expected return on plan assets	(1)	(1)
Amortization of unrecognized prior service benefit	(2)	(6)
Net periodic benefit cost	(2)	(6)
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	5
Interest cost	32	37
Expected return on plan assets	(48)	(53)
Amortization of unrecognized net loss	11	7
Net periodic benefit cost	$ (1)	$ (4)