Stock options and restricted common stock
 12 Months Ended
Dec. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Stock options and restricted common stock
Stock options and restricted common stock
The Company's board of directors adopted and its shareholders approved the 2013 Equity Incentive Plan (the “2013 Plan”), under which the Company approved option grants to its directors and executive officers. The 2013 Plan and these option grants became effective on the closing of the IPO. As of that same date, the Company discontinued the granting of options under the 2008 Equity Incentive Plan (the "2008 Plan").
The options are contingent on the participants' continued employment or other service and are subject to forfeiture if employment terminates for any reason. To date, the Company has granted both incentive stock options and non-qualified stock options. The 2008 Plan and 2013 Plan also provide certain employees accelerated vesting in the event of changes in control.
The Company utilizes the Black-Scholes option-pricing model to determine the fair value of option grants. When the option grants have the characteristics of a "plain-vanilla" grant the expected term is based on the simplified method as prescribed by Staff Accounting Bulletin ("SAB") No. 107 and SAB No. 110. In the event that the options do not have such characteristics the Company bases the expected term on items such as expiration dates and other factors unique to the grantee. The risk-free interest rate assumptions are based on the U.S. Treasury bond rates appropriate for the expected term in effect at the time of grant. The expected volatility is based on calculated enterprise value volatilities for publicly traded companies in the same industry and general stage of development. The forfeiture rates are based on historical experience for similar classes of employees. The dividend yield is based on expected dividends at the time of grant. Prior to the Company's IPO, the determinations of fair values were based on, among other things, valuation analyses, review of historical financial results, business milestones, financial forecasts and business outlook as of the grant dates. Total enterprise value ranges and the total equity value ranges were estimated utilizing both the income approach and the market approach guidelines. Upon completion of the IPO the common stock was valued by reference to the Company's publicly traded price.
Assumptions used to value the most recent option grants were as follows:
 
2012
 
2013
Estimated expected term
6 years

 
6 to 8 years

Risk-free interest rate
0.98
%
 
1.70% to 2.22%

Expected volatility assumption
66
%
 
64% to 68%

Forfeiture rate
0 to 7.0%

 
0 to 7.0%

Dividend yield assumption
—
%
 
—
%

Option awards to consultants, advisors or other independent contractors have been granted with an exercise price equal to the market price of the Company’s stock at the date of the grant, with 10-year contractual terms and vesting dependent upon whether the individual continues to provide services to the Company. The value of stock options to these parties is measured at the then-current market value as of financial reporting dates and compensation cost is recognized for the net change in the fair value of the options for the reporting period, until vesting terms are met. In determining the fair value of options granted to consultants, advisors and other independent contractors, the Company uses the Black-Scholes valuation model in a manner consistent with its use in determining the fair value of options granted to employees and directors. The options that vest are adjusted, for the last time, to the then-current fair value and compensation cost is recognized accordingly.
The number of shares available for grant under the Company's Equity Incentive Plans, as amended (the "Plans") were as follows:
 
2008 Plan
 
2013 Plan
 
Total
Available for grant — January 1, 2011
175,984

 
—

 
175,984

Plan amendment
461,538

 
—

 
461,538

Grants
(421,694
)
 
—

 
(421,694
)
Forfeitures
27,837

 
—

 
27,837

Available for grant — December 31, 2011
243,665

 
—

 
243,665

Plan adoptions and amendments
262,256

 
—

 
262,256

Grants
(99,187
)
 
—

 
(99,187
)
Forfeitures
6,954

 
—

 
6,954

Available for grant — December 31, 2012
413,688

 
—

 
413,688

Plan adoptions and amendments
(421,371
)
 
831,659

 
410,288

Grants
—

 
(812,116
)
 
(812,116
)
Forfeitures
7,683

 
10,327

 
18,010

Available for grant — December 31, 2013
—

 
29,870

 
29,870


A summary of option activity under the Plans as of December 31, 2013 and changes during the period then ended were as follows:
 
Options
 
Weighted-
average
exercise
price
 
Weighted-
average
remaining
contractual
term
Outstanding — January 1, 2011
1,156,301

 
$
6.44

 
8.88
Granted
421,694

 
12.68

 
 
Exercised
(4,220
)
 
6.24

 
 
Forfeited
(27,837
)
 
8.78

 
 
Outstanding — December 31, 2011
1,545,938

 
8.09

 
8.42
Granted
99,187

 
12.68

 
 
Exercised
(20,296
)
 
7.41

 
 
Forfeited
(6,954
)
 
12.19

 
 
Outstanding — December 31, 2012
1,617,875

 
8.39

 
7.63
Granted
812,116

 
12.31

 
 
Exercised
(23,587
)
 
6.55

 
 
Forfeited
(18,010
)
 
11.34

 
 
Outstanding — December 31, 2013
2,388,394

 
$
9.70

 
7.63
 
 
 
 
 
 
Vested as of December 31, 2013
1,294,392

 
$
7.67

 
6.39
 
 
 
 
 
 
Vested or expected to vest as of December 31, 2013
2,379,773

 
$
9.69

 
7.63
 
 
 
 
 
 
Exercisable as of December 31, 2013
1,627,426

 
$
8.47

 
6.70
 
 
 
 
 
 

Under the terms of the 2008 Plan, option grants are immediately exercisable. If a participant exercises unvested options, the shares received from the exercise are considered restricted shares and the participant's rights thereto remain subject to the original vesting schedule of the options. At December, 31, 2012 and 2013, there were no such restricted shares. Under the terms of the 2013 Plan, options are not exercisable until vested.
The weighted-average grant-date fair value of stock options granted during 2011, 2012 and 2013 was $3.41, $4.10 and $8.31 per share, respectively. The aggregate intrinsic value of outstanding stock options was $16.3 million as of December 31, 2013. At December 31, 2013 total compensation cost not yet recognized that related to nonvested awards was $6.7 million, which was expected to be recognized over a weighted-average period of 3.03 years. That amount excludes options issued in exchange for and accounted as intangible assets.