Employee Benefits - Components of Net Pension and Other Postretirement Benefit Costs Reflected in Net Income (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Benefits [Member]
Components of net pension and other postretirement benefit costs reflected in net income
Service cost	$ 86	$ 95	$ 88
Interest cost	140	125	124
Expected return on plan assets	(186)	(165)	(155)
Amortization of net actuarial loss and prior service cost and other	35	90	80
Total benefit costs reflected in net income	75	145	137
Other Postretirement Benefits [Member]
Components of net pension and other postretirement benefit costs reflected in net income
Service cost	10	13	12
Interest cost	19	19	20
Expected return on plan assets	(9)	(7)	(6)
Amortization of net actuarial loss and prior service cost and other		3	3
Total benefit costs reflected in net income	$ 20	$ 28	$ 29