Stock-Based Compensation - Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Number of shares
Outstanding, beginning balance (in shares)	3,349	3,205
Granted (in shares)	762	764
Exercised (in shares)	(670)	(527)
Canceled (in shares)	(75)	(93)
Outstanding, ending balance (in shares)	3,366	3,349
Exercisable (in shares)	1,431
Weighted-average exercise price per share
Outstanding, beginning balance (in dollars per share)	$ 219.40	$ 180.69
Granted (in dollars per share)	333.45	320.65
Exercised (in dollars per share)	157.85	122.94
Canceled (in dollars per share)	304.56	273.64
Outstanding, ending balance (in dollars per share)	255.32	$ 219.40
Exercisable (in dollars per share)	$ 178.23
Weighted-average contractual term, outstanding	6 years 9 months 14 days	6 years 7 months 28 days
Weighted-average contractual term, exercisable	4 years 9 months
Aggregate intrinsic value, outstanding	$ 591.7	$ 452.8
Aggregate intrinsic value, exercisable	$ 361.9

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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- Definition

The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.