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Stock-Based Compensation - Assumptions (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2017 Rate	Sep. 30, 2016 Rate	Sep. 30, 2017 Rate	Sep. 30, 2016 Rate
Employee stock options
Weighted-Average Assumptions
Expected term (in years)	6 years 1 month 6 days	6 years 1 month 6 days
Risk-free interest rate, min (as percent)	1.90%	1.20%	1.90%	1.20%
Risk-free interest rate, max (as percent)	2.10%	1.30%	2.10%	1.90%
Expected volatility, min (as percent)	38.90%	44.40%	38.90%	44.40%
Expected volatility, max (as percent)	39.10%	44.60%	43.80%	45.30%
Employee stock options | Minimum
Weighted-Average Assumptions
Expected term (in years)			6 years	6 years
Employee stock options | Maximum
Weighted-Average Assumptions
Expected term (in years)			6 years 1 month 6 days	6 years 1 month 6 days
Employee Stock Purchase Plan
Weighted-Average Assumptions
Expected term (in years)	6 months		6 months
Risk-free interest rate (as percent)	1.20%	0.00%	1.20%	0.00%
Expected volatility (as percent)	28.50%	0.00%	28.50%	0.00%

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- 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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(ii)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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.

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- Definition

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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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(iv)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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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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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

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