Stockholders' Equity, Stock-based Valuation Information (Details)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	1.80%	1.50%	1.50%
Dividend yield	0.00%	0.00%	0.00%
Volatility	65.90%	58.70%	53.80%
Expected life	4 years 6 months	4 years 6 months	4 years 6 months
Board of Director Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	2.20%	1.30%	2.10%
Dividend yield	0.00%	0.00%	0.00%
Volatility	61.20%	53.10%	52.20%
Expected life	6 years 7 months 6 days	6 years 6 months	6 years 10 months 24 days
Employee Stock Purchase Plan (ESPP) [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	0.80%	0.40%	0.10%
Dividend yield	0.00%	0.00%	0.00%
Volatility	59.90%	86.40%	51.70%
Expected life	6 months	6 months	6 months

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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.

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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

Weighted average expected volatility rate 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://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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