Income Taxes - Unrecognized Tax Benefits (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Reconciliation of Unrecognized Tax Benefits
Gross unrecognized tax benefits-beginning of period	$ 164	$ 63	$ 18
Increases related to prior year tax positions	425	13	5
Decreases related to prior year tax positions	(13)	(16)	(2)
Increases related to current year tax positions	740	104	42
Gross unrecognized tax benefits-end of period	$ 1,316	$ 164	$ 63