Postretirement Benefits (Details 4) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Projected benefit obligation:
Benefit obligation at beginning of period	$ 488	$ 400
Service cost for benefits earned	21	17	15
Interest cost on benefit obligation	24	23	21
Plan amendments	0	1
Actuarial loss	87	54
Benefits paid	(8)	(7)
Benefit obligation at end of period	612	488	400
Accumulated benefit obligation at end of period	$ 453	$ 363