Share-Based Compensation - Stock Options Activity (Details) - Stock Option - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Shares
Outstanding at January 1, 2018 (in shares)	1,924,683
Granted (in shares)	314,895
Exercised (in shares)	(455,558)
Forfeited (in shares)	(70,017)
Outstanding at December 31, 2018(in shares)	1,714,003	1,924,683
Vested and expected to vest at December 31, 2018 (in shares)	1,635,247
Exercisable at December 31, 2018 (in shares)	836,917
Weighted Average Price
Outstanding at January 1, 2018 (in dollars per share)	$ 71.07
Granted (in dollars per share)	138.40
Exercised (in dollars per share)	60.81
Forfeited (in dollars per share)	97.68
Outstanding at December 31, 2018 (in dollars per share)	85.08	$ 71.07
Vested and expected to vest at December 31, 2018 (in dollars per share)	83.77
Exercisable at December 31, 2018 (in dollars per share)	$ 66.52
Weighted-Average Remaining Contractual Term
Weighted-Average Remaining Contractual Term	6 years 8 months 12 days	6 years 10 months 13 days
Vested and expected to vest at December 31, 2018	6 years 7 months 10 days
Exercisable at December 31, 2018	5 years 3 months 4 days
Aggregate Intrinsic Value
Outstanding at January 1, 2018	$ 117,209,218
Outstanding at December 31, 2018	74,191,783	$ 117,209,218
Vested and expected to vest at December 31, 2018	72,632,325
Exercisable at December 31, 2018	$ 50,027,326

Share Based Compensation Arrangement By Share Based Payment Award Option Exercised

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Options, Aggregate Intrinsic Value [Abstract]

+ References

No definition available.

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Share Based Compensation Arrangement By Share Based Payment Award Options Granted

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No definition available.

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Options, Weighted Average Remaining Contractual Term [Abstract]

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No definition available.

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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.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iii)
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- Definition

For presentations that combine terminations, 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 or that expired.

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- Definition

Weighted average price of options that were either forfeited or expired.

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- Definition

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

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- Definition

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

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- Definition

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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- Definition

Number of fully vested and expected to vest exercisable options that may 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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- Definition

Amount by which current fair value of underlying stock exceeds exercise price of 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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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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- Definition

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

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No definition available.

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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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No definition available.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. 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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