Corning maintains long-term incentive plans (the Plans) for key employees and non-employee members of our Board of Directors. The Plans allow us to grant equity-based compensation awards, including stock options, stock appreciation rights, performance share units, restricted stock units, restricted stock awards or a combination of awards (collectively, share-based awards). At December 31, 2015, there were approximately 72 million unissued common shares available for future grants under the Plans.

The fair value of awards granted subsequent to January 1, 2006 that are expected to ultimately vest is recognized as expense over the requisite service periods. The number of options expected to vest equals the total options granted less an estimation of the number of forfeitures expected to occur prior to vesting. The forfeiture rate is calculated based on 15 years of historical data and is adjusted if actual forfeitures differ significantly from the original estimates. The effect of any change in estimated forfeitures would be recognized through a cumulative adjustment that would be included in compensation cost in the period of the change in estimate.

Total share-based compensation cost of $46 million, $58 million and $54 million was disclosed in operating activities on the Company’s Consolidated Statements of Cash Flows for the years ended December 31, 2015, 2014 and 2013, respectively.

Corning’s stock option plans provide non-qualified and incentive stock options to purchase authorized but unissued shares, or treasury shares, at the market price on the grant date and generally become exercisable in installments from one to five years from the grant date. The maximum term of non-qualified and incentive stock options is 10 years from the grant date.

The aggregate intrinsic value (market value of stock less option exercise price) in the preceding table represents the total pretax intrinsic value, based on the Company’s closing stock price on December 31, 2015, which would have been received by the option holders had all option holders exercised their “in-the-money” options as of that date. The total number of “in-the-money” options exercisable on December 31, 2015, was approximately 13 million.

The weighted-average grant-date fair value for options granted for the years ended December 31, 2015, 2014 and 2013 was $7.99, $8.29 and $5.02, respectively. The total fair value of options that vested during the years ended December 31, 2015, 2014 and 2013 was approximately $36 million, $16 million and $29 million, respectively. Compensation cost related to stock options for the years ended December 31, 2015, 2014 and 2013, was approximately $14 million, $22 million and $25 million, respectively.

As of December 31, 2015, there was approximately $7 million of unrecognized compensation cost related to stock options granted under the Plans. The cost is expected to be recognized over a weighted-average period of 1.7 years.

Proceeds received from the exercise of stock options were $102 million for the year ended December 31, 2015, which were included in financing activities on the Company’s Consolidated Statements of Cash Flows. The total intrinsic value of options exercised for the years ended December 31, 2015, 2014 and 2013 was approximately $48 million, $69 million and $55 million, respectively. The income tax benefit realized from share-based compensation was not significant for the years ended December 31, 2015 and 2014. There were no income tax benefits realized from share-based compensation for the year ended December 31, 2013, due to net operating loss and credit carryforwards available to the Company. Refer to Note 6 (Income Taxes).

An award is considered vested when the employee’s retention of the award is no longer contingent on providing subsequent service (the “non-substantive vesting period approach”). Awards to retirement eligible employees are fully vested at the date of grant, and the related compensation expense is recognized immediately upon grant or over the period from the grant date to the date of retirement eligibility for employees that become age 55 during the vesting period.

Corning uses a multiple-point Black-Scholes valuation model to estimate the fair value of stock option grants. Corning utilizes a blended approach for calculating the volatility assumption used in the multiple-point Black-Scholes valuation model defined as the weighted average of the short-term implied volatility, the most recent volatility for the period equal to the expected term, and the most recent 15-year historical volatility. The expected term assumption is the period of time the options are expected to be outstanding, and is calculated using a combination of historical exercise experience adjusted to reflect the current vesting period of options being valued, and partial life cycles of outstanding options. The risk-free rates used in the multiple-point Black-Scholes valuation model are the implied rates for a zero-coupon U.S. Treasury bond with a term equal to the option’s expected term. The ranges given below reflect results from separate groups of employees exhibiting different exercise behavior.

The Corning Incentive Stock Plan permits restricted stock and restricted stock unit grants, either determined by specific performance goals or issued directly, in most instances, subject to the possibility of forfeiture and without cash consideration. Restricted stock and restricted stock units under the Incentive Stock Plan are granted at the closing market price on the grant date, contingently vest over a period of generally one to ten years, and generally have contractual lives of one to ten years. The fair value of each restricted stock grant or restricted stock unit awarded under the Incentive Stock Plan is based on the grant date closing price of the Company’s stock.

Time-based restricted stock and restricted stock units are issued by the Company on a discretionary basis, and are payable in shares of the Company’s common stock upon vesting. The fair value is based on the closing market price of the Company’s stock on the grant date. Compensation cost is recognized over the requisite vesting period and adjusted for actual forfeitures before vesting.

The following table represents a summary of the status of the Company’s non-vested time-based restricted stock and restricted stock units as of December 31, 2014, and changes which occurred during the year ended December 31, 2015:

As of December 31, 2015, there was approximately $27 million of unrecognized compensation cost related to non-vested time-based restricted stock and restricted stock units compensation arrangements granted under the Plan. The cost is expected to be recognized over a weighted-average period of 2.3 years. The total fair value of time-based restricted stock that vested during the years ended December 31, 2015, 2014 and 2013 was approximately $32 million, $32 million and $29 million, respectively. Compensation cost related to time-based restricted stock and restricted stock units was approximately $32 million, $36 million and $29 million for the years ended December 31, 2015, 2014 and 2013, respectively.