Components of Net Periodic Pension and Postretirement Benefit Costs (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Pension benefits
Components of net periodic cost
Service cost	$ 39	$ 38	$ 79	$ 76
Interest cost	66	75	132	149
Expected return on plan assets	(96)	(99)	(193)	(197)
Amortization of prior service credit	(1)	(1)	(1)	(1)
Amortization of net actuarial loss (gain)	65	45	130	89
Settlement loss	9	9	19	19
Net periodic cost (credit)	82	67	166	135
Postretirement benefits
Components of net periodic cost
Service cost	3	3	7	6
Interest cost	9	9	17	18
Amortization of prior service credit	(6)	(5)	(12)	(11)
Amortization of net actuarial loss (gain)	(3)	(5)	(6)	(10)
Net periodic cost (credit)	$ 3	$ 2	$ 6	$ 3