Stock-Based Compensation Plans - Summary of Stock Options Activity (Details) - Amended and Restated 2004 Stock Incentive Plan $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Shares Subject to Options (in shares)
Outstanding as of beginning of the period | shares	6,985
Granted | shares	345
Exercised | shares	(2,431)
Forfeited | shares	(133)
Expired | shares	0
Outstanding as of end of the period | shares	4,766
Exercisable as of end of the period | shares	3,992
Weighted- Average Exercise Price (in dollars per share)
Outstanding as of beginning of the period | $ / shares	$ 48.03
Granted | $ / shares	86.34
Exercised | $ / shares	51.04
Forfeited | $ / shares	75.48
Expired | $ / shares	0.00
Outstanding as of end of the period | $ / shares	48.50
Exercisable as of end of the period | $ / shares	$ 43.33
Weighted-Average Remaining Contractual Term, Outstanding at end of period, years	4 years 1 month
Weighted-Average Remaining contractual Term Exercisable, at end of period years	3 years 3 months
Aggregate Intrinsic Value, Outstanding, at end of period | $	$ 243
Aggregate Intrinsic Value, Exercisable, at end of period | $	$ 225