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, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits [Member]
Components of net pension and other postretirement benefit costs reflected in net income
Service cost	$ 88	$ 79	$ 75
Interest cost	124	120	112
Expected return on plan assets	(155)	(140)	(131)
Amortization of net actuarial loss and prior service cost and other	80	68	64
Total benefit costs reflected in net income	137	127	120
Other Postretirement Benefits [Member]
Components of net pension and other postretirement benefit costs reflected in net income
Service cost	12	11	11
Interest cost	20	22	21
Expected return on plan assets	(6)	(5)	(4)
Amortization of net actuarial loss and prior service cost and other	3	3	2
Total benefit costs reflected in net income	$ 29	$ 31	$ 30