Employee Benefits - Net Actuarial Loss (Gain) and Prior Service Cost Included in Accumulated Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	$ 842	$ 396
Prior service cost	14	16
Total net actuarial loss and prior service cost included in accumulated other comprehensive income	856	412
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	58	(13)
Prior service cost	1	1
Total net actuarial loss and prior service cost included in accumulated other comprehensive income	$ 59	$ (12)