Share-Based Compensation - Stock Option Activities (Details) - Stock Option Plan - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock option activity, shares
Stock options at the beginning of the period (in shares)	20,544
Stock options, granted (in shares)	3,364
Stock options, exercised (in shares)	(4,161)
Stock options, forfeited (in shares)	(2,543)
Stock options, expired (in shares)	(67)
Stock options at the ending of the period (in shares)	17,137	20,544
Stock options, vested and expected to vest (in shares)	14,589
Stock options, exercisable (in shares)	6,120
Weighted-average exercise price
Stock options, weighted-average strike price at the beginning of the period (in dollars per share)	$ 34.54
Stock options, weighted-average exercise price, granted (in dollars per share)	55.69
Stock options, weighted-average exercise price, exercised (in dollars per share)	23.67
Stock options, weighted-average exercise price, forfeited (in dollars per share)	45.07
Stock options, weighted-average exercise price, expired (in dollars per share)	43.50
Stock options, weighted-average strike price at the end of the period (in dollars per share)	39.73	$ 34.54
Stock options, weighted-average exercise price, vested and expected to vest (in dollars per share)	38.50
Stock options, weighted-average exercise price, exercisable (in dollars per share)	$ 28.73
Stock options, weighted-average remaining contractual term	6 years 9 months 21 days
Stock options, weighted-average remaining contractual term, vested and expected to vest	6 years 11 months 22 days
Stock options, weighted-average remaining contractual term, exercisable	5 years 7 months 6 days
Stock options, aggregate intrinsic value	$ 185
Stock options, aggregate intrinsic value, vested and expected to vest	172
Stock options, aggregate intrinsic value, exercisable	116
Stock options, intrinsic value of options exercised	196	$ 372	$ 161
Total grant date fair value of options vested	45	$ 47	$ 40
Share-based compensation, unrecognized compensation	$ 84
Share-based compensation, unrecognized compensation weighted-average period of recognition	1 year 2 months 12 days