Stock Options, Restricted Stock and Employee Stock Purchase Plan - Weighted-average Assumptions for Stock Options (Details) - $ / shares	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assumptions used in estimating fair value of stock options
Expected life (in years)	4 years 9 months	4 years 8 months 23 days	4 years 9 months 18 days
Dividend yield	0.00%	0.00%	0.00%
Fair value per stock option (in dollars per share)	$ 9.25	$ 11.13	$ 14.59
Minimum
Assumptions used in estimating fair value of stock options
Expected volatility	50.00%	46.00%	42.00%
Risk-free interest rate	1.70%	0.90%	1.50%
Maximum
Assumptions used in estimating fair value of stock options
Expected volatility	50.00%	50.00%	47.00%
Risk-free interest rate	1.70%	1.80%	1.60%

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.

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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.

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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 weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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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.

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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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