Aggregate number of common shares reserved for future issuance.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

The estimated measure of the maximum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The estimated measure of the minimum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The maximum risk-free interest rate assumption that is used in valuing an option on its own shares.

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The minimum risk-free interest rate assumption that is used in valuing an option on its own shares.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Percentage of vesting of share-based compensation awards.

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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Employee Benefits and Share-Based Compensation - Stock Award Plans (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance (in shares)	8,847
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life, years	4 years 9 months 15 days	4 years 8 months 1 day	4 years 10 months 24 days
2009 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance (in shares)	6,900
2009 Plan \| Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance (in shares)	3,748
Vesting period (in years)	4 years
2009 Plan \| Employee Stock Option \| Vesting one year from the vesting commencement date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)	1 year
Percentage of award vesting	25.00%
2009 Plan \| Employee Stock Option \| Vesting in equal monthly installments
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)	36 months
2009 Plan \| Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance (in shares)	755
Vesting period (in years)	4 years
2009 Plan \| Restricted Stock Units (RSUs) \| Vesting one year from the vesting commencement date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)	1 year
Percentage of award vesting	25.00%
2009 Plan \| Restricted Stock Units (RSUs) \| Vesting in equal monthly installments
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)	3 years
1997 Plan \| Employee Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
ESPP Risk-free interest rate, minimum	0.80%	0.50%	0.30%
ESPP Risk-free interest rate, maximum	2.70%	1.40%	0.80%
Expected volatility, minimum	28.10%	25.80%	25.80%
Expected volatility, maximum	33.80%	32.80%	34.80%
Min \| 1997 Plan \| Employee Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life, years	6 months	6 months	6 months
Max \| 1997 Plan \| Employee Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life, years	2 years	2 years	2 years