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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2014
Stock-Based Compensation |
Summary of Founders' stock activity and related information |
|  | Shares |
Unvested at December 31, 2012 |  |  | 1,645,141 |
Granted |  |  | — |
Vested |  |  | (658,060 | )
​ | ​ | ​ | ​ | ​
Unvested at December 31, 2013 |  |  | 987,081 |
Granted |  |  | — |
Vested |  |  | (658,060 | )
​ | ​ | ​ | ​ | ​
Unvested at December 31, 2014 |  |  | 329,021 |
Summary of stock option activity and related information | A summary of the Company's stock option activity and related information is as follows (in thousands, except share and per share data):
|  | Shares |  | Weighted average exercise price per share |  | Weighted average remaining contractual term (years) |  | Aggregate intrinsic value |
Outstanding at December 31, 2013 |  |  | 2,413,237 |  | $ | 9.26 |  |  | 9.05 |  | $ | 6,087 |
Granted |  |  | 2,464,138 |  |  | 19.12 |  |  |  |  |  |  |
Exercised |  |  | (370,808 | ) |  | 3.81 |  |  |  |  |  |  |
Forfeited |  |  | (852,564 | ) |  | 11.51 |  |  |  |  |  |  |
Cancelled |  |  | (25,375 | ) |  | 5.48 |  |  |  |  |  |  |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Outstanding at December 31, 2014 |  |  | 3,628,628 |  |  | 16.01 |  |  | 9.11 |  |  | 102,355 |
Exercisable at December 31, 2014 |  |  | 688,562 |  |  | 8.43 |  |  | 8.07 |  |  | 24,641 |
Vested and expected to vest at December 31, 2014 |  |  | 2,394,044 |  |  | 15.11 |  |  | 9.01 |  |  | 69,692 |
Schedule of assumptions used to estimate fair value of each stock-based option award on the grant date using the Black-Scholes option pricing model |
|  | December 31,
|  | 2014 |  | 2013 |  | 2012
Risk-free interest rate |  | 1.62% - 2.17% |  | 0.91% - 2.11% |  | 0.8% - 1.78%
Dividend yield |  | — |  | — |  | —
Volatility |  | 76% - 84% |  | 83% - 91% |  | 79% - 89%
Expected term (years) |  | 5.3 - 9.97 |  | 5.1 - 9.93 |  | 5.1 - 9.93

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- Definition Tabular disclosure of the number and weighted-average exercise prices (or conversion ratios) for share options (or share units) that were outstanding at the beginning and end of the year, vested and expected to vest, exercisable or convertible at the end of the year, and the number of share options or share units that were granted, exercised or converted, forfeited, and expired during the year. + 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 (c)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + 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) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the change in restricted stock units (RSUs). + References No definition available. + Details
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