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, 2018 USD ($) $ / shares shares
Shares Subject to Options (in shares)
Outstanding as of beginning of the period | shares	4,766
Granted | shares	0
Exercised | shares	(1,310)
Forfeited | shares	0
Expired | shares	0
Outstanding as of end of the period | shares	3,456
Exercisable as of end of the period | shares	3,043
Weighted- Average Exercise Price (in dollars per share)
Outstanding as of beginning of the period | $ / shares	$ 48.50
Granted | $ / shares	0.00
Exercised | $ / shares	28.65
Forfeited | $ / shares	0.00
Expired | $ / shares	0.00
Outstanding as of end of the period | $ / shares	56.03
Exercisable as of end of the period | $ / shares	$ 53.36
Weighted-Average Remaining Contractual Term, Outstanding at end of period, years	3 years 10 months 17 days
Weighted-Average Remaining contractual Term Exercisable, at end of period years	3 years 4 months 13 days
Aggregate Intrinsic Value, Outstanding, at end of period | $	$ 71
Aggregate Intrinsic Value, Exercisable, at end of period | $	$ 68