Employee Benefits - Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Benefits [Member]
Changes in plan assets and benefit obligations recognized in other comprehensive income
Net actuarial loss (gain)	$ 479	$ (505)	$ 139
Amortization of net actuarial loss and prior service cost and other	(35)	(90)	(80)
Total changes in plan assets and benefit obligations recognized in other comprehensive income	444	(595)	59
Other Postretirement Benefits [Member]
Changes in plan assets and benefit obligations recognized in other comprehensive income
Net actuarial loss (gain)	71	(117)	(11)
Amortization of net actuarial loss and prior service cost and other		(3)	(3)
Total changes in plan assets and benefit obligations recognized in other comprehensive income	$ 71	$ (120)	$ (14)