Restructuring Costs (Restructuring Activities) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Restructuring Reserve [Roll Forward]
Liability at beginning of period	$ 170	$ 181
Amount Accrued	7	78
Cash Paid	(29)	(88)
Other	(5)	(1)
Liability at end of period	143	170
Severance [Member]
Restructuring Reserve [Roll Forward]
Liability at beginning of period	36	27
Amount Accrued	4	46
Cash Paid	(17)	(38)
Other	(3)	1
Liability at end of period	20	36
Future Rent Under Non-Cancelable Leases and Other Costs [Member]
Restructuring Reserve [Roll Forward]
Liability at beginning of period	134	154
Amount Accrued	3	32
Cash Paid	(12)	(50)
Other	(2)	(2)
Liability at end of period	$ 123	$ 134