Employee Stock and Savings Plans - Summary of Stock Option Activity (Details) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Shares
Stock options outstanding - balance at beginning of period (in shares) | shares	7,743
Granted (in shares) | shares	717
Forfeited (in shares) | shares	(108)
Exercised (in shares) | shares	(1,457)
Stock options outstanding - balance at end of period (in shares) | shares	6,895
Stock options exercisable (in shares) | shares	5,249
Weighted- Average Exercise Price
Stock options outstanding - balance at beginning of period (in dollars per share) | $ / shares	$ 48.03
Granted (in dollars per share) | $ / shares	114.76
Forfeited (in dollars per share) | $ / shares	95.02
Exercised (in dollars per share) | $ / shares	38.07
Stock options outstanding - balance at end of period (in dollars per share) | $ / shares	56.34
Stock options exercisable (in dollars per share) | $ / shares	$ 42.38
Weighted-Average Remaining Contractual Term and Aggregate Intrinsic Value
Weighted-Average Remaining Contractual Term, Stock options outstanding	5 years 3 months
Weighted-Average Remaining Contractual Term, Stock options exercisable	4 years 3 months
Aggregate Intrinsic Value, Stock options outstanding | $	$ 516
Aggregate Intrinsic Value, Stock options exercisable | $	$ 466