Employee Benefits and Share-Based Compensation - Valuation of Share Based Awards, Table (Details)	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
1997 Employee stock purchase plan \| Employee stock purchase plan (ESPP)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield, %	0.00%	0.00%	0.00%	0.00%
Expected volatility, %, Min	28.10%	26.70%	27.70%	25.80%
Expected volatility, %, Max	33.80%	32.10%	33.80%	32.80%
Risk free interest rate, %, Min	0.80%	0.60%	0.70%	0.50%
Risk free interest rate, %, Max	2.70%	1.40%	2.70%	1.40%
1997 Employee stock purchase plan \| Employee stock purchase plan (ESPP) \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life (in years)	6 months	6 months	6 months	6 months
1997 Employee stock purchase plan \| Employee stock purchase plan (ESPP) \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life (in years)	2 years	2 years	2 years	2 years
2009 Plan \| Employee stock option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life (in years)	4 years 9 months 18 days	4 years 8 months 12 days	4 years 9 months 18 days	4 years 8 months 12 days
Expected volatility, %	30.40%	28.10%	31.10%	29.20%
Risk free interest rate, %	2.80%	1.80%	2.70%	1.80%
Estimated forfeiture rate, %	6.90%	7.70%	6.90%	7.70%
Dividend yield, %	0.00%	0.00%	0.00%	0.00%

Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Estimated Forfeiture Rate

No definition available.

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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

+ References

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- Definition

The estimated measure of the 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.

+ References

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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.

+ References

No definition available.

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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.

+ References

No definition available.

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- Definition

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

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- Definition

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

+ References

No definition available.

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- Definition

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

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No definition available.

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- Definition

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

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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-Publisher FASB
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-Section S99
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-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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