Income Tax (Details 6) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reconciliation of Unrecognized Tax Benefits
Balance at beginning of period	$ 679	$ 810	$ 773
Additions for tax positions of prior years	105	30	186
Reductions for tax positions of prior years	(82)	(161)	(84)
Additions for tax positions of current year	32	13	13
Reductions for tax positions of current year	(9)	(8)	(8)
Settlements with tax authorities	(15)	(5)	(59)
Lapses of statutes of limitations	(2)	0	(11)
Balance at end of the period	708	679	810
Unrecognized tax benefits that, if recognized would impact the effective rate	$ 566	$ 527	$ 536