Employee Benefit Plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	$ 1,441	$ 1,267
Service cost	39	41	35
Interest cost	55	64	64
Plan participants' contributions	0	0
Benefits paid	(75)	(66)
Medicare Part D and Early Retiree Reinsurance Program subsidies	0	0
Plan amendment	0	0
Actuarial loss (gain)	98	143
Settlements	(9)	(8)
Benefit obligation at end of year	1,549	1,441	1,267
Other Postretirement Benefits [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	339	289
Service cost	3	2	2
Interest cost	13	15	15
Plan participants' contributions	5	6
Benefits paid	(20)	(22)
Medicare Part D and Early Retiree Reinsurance Program subsidies	3	4
Plan amendment	(6)	(3)
Actuarial loss (gain)	(6)	48
Settlements	0	0
Benefit obligation at end of year	$ 331	$ 339	$ 289