Stock-Based Compensation - Summary of Changes in Number of Shares of Common Stock Under Option (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares at beginning of period (in shares)	8,031
Granted (in shares)	408
Exercised (in shares)	(692)
Forfeited (in shares)	(40)
Shares at end of period (in shares)	7,707
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted average exercise price, beginning of period (in dollars per share)	$ 109.78
Weighted average exercise price, granted (in dollars per share)	204.57
Weighted average exercise price, exercised (in dollars per share)	71.50
Weighted average exercise price, forfeitures (in dollars per share)	149.22
Weighted average exercise price, end of period (in dollars per share)	$ 118.03
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Expected to vest (in shares)	7,707
Weighted average exercise price, expected to vest (in dollars per share)	$ 118.03
Options exercisable at beginning of period (in shares)	4,588	4,029
Weighted average exercise price of exercisable options, beginning of period (in dollars per share)	$ 91.27	$ 75.80
Weighted average exercise price of granted options (in dollars per share)	$ 50.04
Aggregate intrinsic value	$ 846,395	$ 663,815
Aggregate intrinsic value, exercised	$ 108,164

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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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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.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Number of share options (or share units) exercised during the current period.

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