3.  STOCK-BASED COMPENSATION

Stock options are granted under our long-term incentive plan and have an exercise price that may not be less than the fair market value of the underlying shares on the date of grant. In general, stock options granted to participants will become exercisable over a period determined by the Compensation Committee of our Board of Directors, which in the past has been a five year term, with the options vesting over a service period of two to five years. A portion of the stock options granted in the six months ended June 30, 2015 and 2014 were vested immediately with the remainder vesting over a two year period. In addition, stock options will become exercisable upon a change in control, unless provided otherwise by the Compensation Committee of our Board of Directors.

We record non-cash compensation expense related to stock-based compensation as general and administrative expense.  For the three and six months ended June 30, 2015, non-cash compensation expense was $0.7 million and $2.3 million, and was $0.6 million and $2.0 million for the same periods of 2014, related to the issuance of stock options and restricted stock. Because we do not pay significant United States federal income taxes, no amounts were recorded for tax benefits.

Stock option activity for the June 30, 2015 is provided below:

In the three months ended March 31, 2015, we withheld 114,985 shares on the exercise of options which would have otherwise been acquired by the holder on exercise having a value equal to the exercise price of the options being exercised. These shares became treasury shares.  We did not add any shares to treasury in the three months ended June 30, 2015 or in the three and six months ended June 30, 2014.

Shares of restricted stock are granted under our long-term incentive plan using the fair market value of the underlying shares on the date of grant. In general, restricted stock granted to employees will vest over a period determined by the Compensation Committee which is generally a three year period, vesting in three equal parts on the first three anniversaries of the date of the grant.

Restricted stock activity for the six months ended June 30, 2015 is provided below: