On April 28, 2014, our stockholders approved the Digital Realty Trust, Inc., Digital Services, Inc., and Digital Realty Trust, L.P. 2014 Incentive Award Plan (as amended, the 2014 Incentive Award Plan). The 2014 Incentive Award Plan became effective and replaced the Amended and Restated 2004 Incentive Award Plan, as amended, as of the date of such stockholder approval. The material features of the 2014 Incentive Award Plan are described in our definitive Proxy Statement filed on March 19, 2014 in connection with the 2014 Annual Meeting of Stockholders, which description is incorporated herein by reference. Effective as of September 14, 2017, the 2014 Incentive Award Plan was amended to provide that shares which remained available for issuance under DFT’s Amended and Restated 2011 Equity Incentive Plan immediately prior to the closing of the DFT Merger (as adjusted and converted into shares of Digital Realty Trust, Inc.’s common stock) may be used for awards under the 2014 Incentive Award Plan and will not reduce the shares authorized for grant under the 2014 Incentive Award Plan, to the extent that using such shares is permitted without stockholder approval under applicable stock exchange rules. In connection with the amendment to the 2014 Incentive Award Plan, on September 22, 2017, Digital Realty Trust, Inc. registered an additional 3.7 million shares that may be issued pursuant to the 2014 Incentive Award Plan.

As of December 31, 2019, approximately 6.6 million shares of common stock, including awards convertible into or exchangeable for shares of common stock, remained available for future issuance under the 2014 Incentive Award Plan. Each long-term incentive unit and each Class D unit issued under the 2014 Incentive Award Plan counts as one share of common stock for purposes of calculating the limit on shares that may be issued under the 2014 Incentive Award Plan and the individual award limits set forth therein.

Below is a summary of our compensation expense for the years ended December 31, 2019, 2018 and 2017 and our unearned compensation as of December 31, 2019 and December 31, 2018 (in millions):

The following table sets forth the weighted average fair value of for each type of incentive award at the date of grant for the years ended December 31, 2019, 2018 and 2017:

Long-term incentive units, which are also referred to as profits interest units, may be issued to eligible participants for the performance of services to or for the benefit of the Operating Partnership. Long-term incentive units (other than Class D units), whether vested or not, will receive the same quarterly per unit distributions as Operating Partnership common units, which equal the per share distributions on Digital Realty Trust, Inc. common stock. Initially, long-term incentive units do not have full parity with common units with respect to liquidating distributions. If such parity is reached, vested long-term incentive units may be converted into an equal number of common units of the Operating Partnership at any time, and thereafter enjoy all the rights and privileges of common units of the Operating Partnership, including redemption rights.

In order to achieve full parity with common units, long-term incentive units must be fully vested and the holder’s capital account balance in respect of such long-term incentive units must be equal to the capital account balance of a holder of an equivalent number of common units. The capital account balance attributable to each common unit is generally expected to be the same, in part because of the amount credited to a partner’s capital account upon the partner’s contribution of property to the Operating Partnership, and in part because the partnership agreement provides, in most cases, that allocations of income, gain, loss and deduction (which will adjust the partner’s capital accounts) are to be made to the common units on a proportionate basis. As a result, with respect to a number of long-term incentive units, it is possible to determine the capital account balance of an equivalent number of common units by multiplying the number of long-term incentive units by the capital account balance with respect to a common unit.

A partner’s initial capital account balance is equal to the amount the partner paid (or contributed to the Operating Partnership) for the partner’s units and is subject to subsequent adjustments, including with respect to the partner’s share of income, gain or loss of the Operating Partnership. Because a holder of long-term incentive units generally will not pay for the long-term incentive units, the initial capital account balance attributable to such long-term incentive units will be zero. However, the Operating Partnership is required to allocate income, gain, loss and deduction to the partner’s capital accounts in accordance with the terms of the partnership agreement, subject to applicable Treasury Regulations. The partnership agreement provides that holders of long-term incentive units will receive special allocations of gain in the event of a sale or “hypothetical sale” of assets of the Operating Partnership prior to the allocation of gain to Digital Realty Trust, Inc. or other limited partners with respect to their common units. The amount of any such allocation will, to the extent of any such gain, be equal to the difference between the capital account balance of a holder of long-term incentive units attributable to such units and the capital account balance attributable to an equivalent number of common units. If and when such gain allocation is fully made, a holder of long-term incentive units will have achieved full parity with holders of common units. To the extent that, upon an actual sale or a “hypothetical sale” of the Operating Partnership’s assets as described above, there is not sufficient gain to allocate to a holder’s capital account with respect to long-term incentive units, or if such sale or “hypothetical sale” does not occur, such units will not achieve parity with common units.

The term “hypothetical sale” refers to circumstances that are not actual sales of the Operating Partnership’s assets but that require certain adjustments to the value of the Operating Partnership’s assets and the partners’ capital account balances. Specifically, the partnership agreement provides that, from time to time, in accordance with applicable Treasury Regulations, the Operating Partnership will adjust the value of its assets to equal their respective fair market values, and adjust the partners’ capital accounts, in accordance with the terms of the partnership agreement, as if the Operating Partnership sold its assets for an amount equal to their value. Such adjustments will generally be made upon the liquidation of the Operating Partnership, the acquisition of an additional interest in the Operating Partnership by a new or existing partner in exchange for more than a de minimis capital contribution, the distribution by the Operating Partnership to a partner of more than a de minimis amount of partnership property as consideration for an interest in the Operating Partnership, the grant of an interest in the Operating Partnership (other than a de minimis interest) as consideration for the performance of services to or for the benefit of the Operating Partnership (including the grant of a long-term incentive unit), and at such other times as may be desirable or required to comply with the Treasury Regulations.

Below is a summary of our long-term incentive unit activity for the year ended December 31, 2019.

The grant date fair values, which equal the market price of Digital Realty Trust, Inc. common stock on the applicable grant date(s), are being expensed on a straight-line basis for service awards between two and four years, the current vesting periods of the long-term incentive units.

During the years ended December 31, 2019, 2018 and 2017, the Compensation Committee of the Board of Directors of Digital Realty Trust, Inc. approved the grant of market performance-based Class D units of the Operating Partnership and market performance-based restricted stock units, or RSUs, covering shares of Digital Realty Trust, Inc.’s common stock (collectively, the “awards”), under the 2014 Incentive Award Plan to officers and employees of the Company.

The awards, which were determined to contain a market condition, utilize total shareholder return, or TSR, over a three-year measurement period as the market performance metric. Awards will vest based on the Company’s TSR relative to the MSCI US REIT Index, or RMS, over a three-year market performance period, or the Market Performance Period, commencing in January 2017, January 2018 or January 2019, as applicable (or, if earlier, ending on the date on which a change in control of the Company occurs), subject to continued services. Vesting with respect to the market condition is measured based on the difference between Digital Realty Trust, Inc.’s TSR percentage and the TSR percentage of the RMS, or the RMS Relative Market Performance. In the event that the RMS Relative Market Performance during the applicable Market Performance Period is achieved at the “threshold,” “target” or “high” level as set forth below, the awards will become vested as to the market condition with respect to the percentage of Class D units or RSUs, as applicable, set forth below:

If the RMS Relative Market Performance falls between the levels specified above, the percentage of the award that will vest with respect to the market condition will be determined using straight-line linear interpolation between such levels.

In January 2020, following the completion of the applicable Market Performance Period, the Compensation Committee determined that the RMS Relative Market Performance fell between the target and high level for the 2017 awards and, accordingly, 137,816 Class D units (including 10,971 distribution equivalent units that immediately vested on December 31, 2019) and 29,141 RSUs performance vested, subject to service-based vesting. On February 27, 2020, 50% of the 2017 awards vested and the remaining 50% will vest on February 27, 2021, subject to continued employment through each applicable vesting date.

In January 2019, following the completion of the applicable Market Performance Period, the Compensation Committee determined that the high level had been achieved for the 2016 awards and, accordingly, 339,317 Class D units (including 31,009 distribution equivalent units that immediately vested on December 31, 2018, upon the high level being achieved) and 56,778 RSUs performance vested, subject to service-based vesting. On February 27, 2019, 50% of the 2016 awards vested and the remaining 50% vested on February 27, 2020.

In January 2018, following the completion of the applicable Market Performance Period, the Compensation Committee determined that the high level had been achieved for the 2015 awards and, accordingly, 363,193 Class D units (including 36,246 distribution equivalent units that immediately vested on December 31, 2017, upon the high level being achieved) and 49,707 RSUs performance vested, subject to service-based vesting. On February 27, 2018, 50% of the 2015 awards vested and the remaining 50% vested on February 27, 2019.

Following the completion of the applicable Market Performance Period, the 2018 awards that satisfy the market condition, if any, will vest 50% on February 27, 2021 and 50% on February 27, 2022, subject to continued employment through each applicable vesting date. Following the completion of the Market Performance Period, the 2019 awards that satisfy the market condition, if any, will vest 50% on February 27, 2022 and 50% on February 27, 2023, subject to continued employment through each applicable vesting date.

Service-based vesting will be accelerated, in full or on a pro rata basis, as applicable, in the event of a change in control, termination of employment by the Company without cause, or termination of employment by the award recipient for good reason, death, disability or retirement, in any case, prior to the completion of the applicable Market Performance Period. However, vesting with respect to the market condition will continue to be measured based on RMS Relative Market Performance during the applicable three-year Market Performance Period (or, in the case of a change in control, shortened Market Performance Period).

The fair values of the awards were measured using a Monte Carlo simulation to estimate the probability of the market vesting condition being satisfied. The Company’s achievement of the market vesting condition is contingent on its TSR over a three-year market performance period, relative to the total shareholder return of the RMS. The Monte Carlo simulation is a probabilistic technique based on the underlying theory of the Black-Scholes formula, which was run for 100,000 trials to determine the fair value of the awards. For each trial, the payoff to an award is calculated at the settlement date and is then discounted to the grant date at a risk-free interest rate. The total expected value of the awards on the grant date was determined by multiplying the average value per award over all trials by the number of awards granted. Assumptions used in the valuations are summarized as follows:

These valuations were performed in a risk-neutral framework, and no assumption was made with respect to an equity risk premium.

As of December 31, 2019, 2,509,963 Class D units and 696,379 market performance-based RSUs had been awarded to our executive officers and other employees. The number of units granted reflects the maximum number of Class D units or market performance-based RSUs, as applicable, which will become vested assuming the achievement of the highest level of RMS Relative Market Performance under the awards and, in the case of the Class D units, also includes distribution equivalent units. The grant date fair value of these awards was approximately $22.3 million, $21.8 million and $19.5 million for the years ended December 31, 2019, 2018 and 2017, respectively. We will recognize compensation expense on a straight-line basis over the expected service period of approximately four years.

Below is a summary of our restricted stock activity for the year ended December 31, 2019.

(1)	All restricted stock awards granted in 2019 are subject only to service conditions.

The grant date fair values, which equal the market price of Digital Realty Trust, Inc. common stock on the grant date, are expensed on a straight-line basis for service awards over the vesting period of the restricted stock, which is generally four years.

(d) 401(k) Plan

We have a 401(k) plan whereby our employees may contribute a portion of their compensation to their respective retirement accounts, in an amount not to exceed the maximum allowed under the Code. The 401(k) plan complies with Internal Revenue Service requirements as a 401(k) safe harbor plan whereby matching contributions made by us are 100% vested. The aggregate cost of our contributions to the 401(k) plan was approximately $5.2 million, $4.8 million, and $4.6 million for the years ended December 31, 2019, 2018 and 2017, respectively.