Benefit Plans - Estimated Amounts to be Recognized from AOCI into Net Periodic Cost (Benefit) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization of net actuarial loss	$ 45
Amortization of prior service credit	(1)
Total estimated amounts to be recognized	44
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization of net actuarial loss	1
Amortization of prior service credit	(10)
Total estimated amounts to be recognized	$ (9)