Income Taxes - Unrecognized Tax Benefits (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Reconciliation of tnrecognized tax benefits
Balance at beginning of year	$ 153	$ 164
Additions based on tax positions related to the current year	8	4
Additions for tax positions of prior years	49	60
Reductions for tax positions of prior years	(27)	(19)
Settlements	(18)	(42)
Reductions for lapse of statute of limitations	0	(14)
Balance at end of year	$ 165	$ 153