Marketable Securities Contractual Maturity Table (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Proceeds from Sale and Maturity of Marketable Securities [Abstract]
Available-for-sale Securities, Sale Proceeds	$ 917	$ 737	$ 2,600	$ 1,700
Available-for-Sale Securities Classified by Contractual Maturity, Amortized Cost [Abstract]
Due in one year or less, amortized cost	12,911		12,911
Due after one year through five years, amortized cost	1,844		1,844
Total contractual maturities of available-for-sale securities, amortized cost	14,755		14,755
Available-for-Sale Securities Classified by Contractual Maturity, Fair Value [Abstract]
Due in one year or less, fair value	12,920		12,920
Due after one year through five years, fair value	1,839		1,839
Total contractual maturities of available-for-sale securities, fair value	14,759		14,759
Unrealized Gain (Loss) on Investments [Abstract]
Cumulative net unrealized gains on available-for-sale securities	234		234		62
Trading Securities, Change in Unrealized Holding Gain (Loss)	$ 75	$ 187	$ (34)	$ 128