Equity-Based Compensation - Weighted average assumptions (Details) - $ / shares	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
VAR Plan \| Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 2 months 12 days	6 years 3 months 18 days	6 years 6 months
Volatility	50.00%	52.50%	52.10%
Risk-free interest rate	2.15%	1.48%	1.70%
Dividend yield	0.00%	0.00%	0.00%
Weighted average fair value per share granted (in dollars per share)	$ 24.19	$ 19.03	$ 15.90
2012 ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)		6 months	6 months
Volatility		59.10%	52.10%
Risk-free interest rate		0.46%
Dividend yield		0.00%	0.00%
2012 ESPP \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate			0.07%
2012 ESPP \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate			0.42%

Agreed-upon price for the exchange of the underlying asset relating to the share-based payment award.

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

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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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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