Stock Based Awards and Other Equity Instruments - Summary of Unrecognized Stock-Based Compensation Expense, Net of Estimated Forfeitures and Weighted Average Period Remaining (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense (net of forfeitures), Stock Options	$ 85
Period of recognition (in years)	2 years 8 months 12 days
Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Period of recognition (in years)	3 years 2 months 12 days
Unrecognized compensation expense (net of forfeitures), RSUs	$ 99