Summary of Significant Accounting Policies ((Weighted-average assumptions) (Details)	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stock Option
Assumptions used to estimate the fair value of stock options
Expected volatility, minimum (as a percent)	51.00%	45.00%	52.00%
Expected volatility, maximum (as a percent)	53.00%	52.00%	57.00%
Risk-free interest rate, minimum (as a percent)	1.42%	1.48%	1.71%
Risk-free interest rate, maximum (as a percent)	1.54%	1.92%	2.00%
Stock Option \| Minimum
Assumptions used to estimate the fair value of stock options
Expected term (in years)	5 years 5 months 19 days	5 years 3 months 7 days	5 years 9 months 7 days
Stock Option \| Maximum
Assumptions used to estimate the fair value of stock options
Expected term (in years)	6 years 26 days	6 years 29 days	6 years 29 days
Employee Stock Purchase Plan
Assumptions used to estimate the fair value of stock options
Expected volatility, minimum (as a percent)	42.00%	42.00%	43.00%
Expected volatility, maximum (as a percent)	58.00%	44.00%	44.00%
Risk-free interest rate, minimum (as a percent)	0.08%	0.08%	0.08%
Risk-free interest rate, maximum (as a percent)	0.85%	0.71%	0.49%
Employee Stock Purchase Plan \| Minimum
Assumptions used to estimate the fair value of stock options
Expected term (in years)	6 months	6 months	7 months 17 days
Employee Stock Purchase Plan \| Maximum
Assumptions used to estimate the fair value of stock options
Expected term (in years)	2 years 1 month 6 days	2 years 1 month 6 days	2 years 1 month 21 days

No definition available.

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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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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 maximum 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

The minimum 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

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=6415400&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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