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Stock Based Compensation | 12 Months Ended
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Dec. 31, 2018
Stock Based Compensation [Abstract] |
Disclosure of Compensation Related Costs, Share-based Payments [Text Block] | Stock Based Compensation Overview MAA accounts for its stock based employee compensation plans in accordance with accounting standards governing stock based compensation. These standards require an entity to measure the cost of employee services received in exchange for an award of an equity instrument based on the award's fair value on the grant date and recognize the cost over the period during which the employee is required to provide service in exchange for the award, which is generally the vesting period. Any liability awards issued are remeasured at each reporting period. MAA’s stock compensation plans consist of a number of incentives provided to attract and retain independent directors, executive officers and key employees. Incentives are currently granted under the Second Amended and Restated 2013 Stock Incentive Plan, or the Stock Plan, which was approved at the 2018 annual meeting of MAA shareholders. The Stock Plan allows for the grant of restricted stock and stock options up to 2,000,000 shares. MAA believes that such awards better align the interests of its employees with those of its shareholders. Compensation expense is generally recognized for service based restricted stock awards using the straight-line method over the vesting period of the shares regardless of cliff or ratable vesting distinctions. Compensation expense for market and performance based restricted stock awards is generally recognized using the accelerated amortization method with each vesting tranche valued as a separate award, with a separate vesting date, consistent with the estimated value of the award at each period end. Additionally, compensation expense is adjusted for actual forfeitures for all awards in the period that the award was forfeited. Compensation expense for stock options is generally recognized on a straight-line basis over the requisite service period. MAA presents stock compensation expense in the Consolidated Statements of Operations in "General and administrative expenses". Total compensation expense under the Stock Plan was $12.9 million, $10.8 million and $12.2 million for the years ended December 31, 2018, 2017 and 2016, respectively. Of these amounts, total compensation expense capitalized was $0.5 million, $0.2 million and $0.7 million for the years ended December 31, 2018, 2017 and 2016, respectively. As of December 31, 2018, the total unrecognized compensation expense was $13.5 million. This cost is expected to be recognized over the remaining weighted average period of 1.1 years. Total cash paid for the settlement of plan shares totaled $2.9 million, $4.8 million and $2.0 million for the years ended December 31, 2018, 2017 and 2016, respectively. Information concerning grants under the Stock Plan is provided below. Restricted Stock In general, restricted stock is earned based on either a service condition, performance condition, or market condition, or a combination thereof, and generally vests ratably over a period from 1 year to 5 years. Service based awards are earned when the employee remains employed over the requisite service period and are valued on the grant date based upon the market price of MAA common stock on the date of grant. Market based awards are earned when MAA reaches a specified stock price or specified return on the stock price (price appreciation plus dividends) and are valued on the grant date using a Monte Carlo simulation. Performance based awards are earned when MAA reaches certain operational goals such as funds from operations, or FFO, targets and are valued based upon the market price of MAA common stock on the date of grant as well as the probability of reaching the stated targets. MAA remeasures the fair value of the performance based awards each balance sheet date with adjustments made on a cumulative basis until the award is settled and the final compensation is known. The weighted average grant date fair value per share of restricted stock awards granted during the years ended December 31, 2018, 2017 and 2016, was $71.85, $84.53 and $73.20, respectively. The following is a summary of the key assumptions used in the valuation calculations for market based awards granted during the years ended December 31, 2018, 2017 and 2016:
|  | 2018 |  | 2017 |  | 2016
Risk free rate |  | 1.61% - 2.14% |  | 0.65% - 1.57% |  | 0.49% - 1.27%
Dividend yield |  | 3.884% |  | 3.573% |  | 3.634%
Volatility |  | 15.05% - 17.18% |  | 20.43% - 21.85% |  | 18.41% - 19.45%
Requisite service period |  | 3 years |  | 3 years |  | 3 years
The risk free rate was based on a zero coupon risk-free rate. The minimum risk free rate was based on a period of 0.25 years for the years ended December 31, 2018, 2017 and 2016. The maximum risk free rate was based on a period of 3 years for the years ended December 31, 2018, 2017 and 2016. The dividend yield was based on the closing stock price of MAA stock on the date of grant. Volatility for MAA was obtained by using a blend of both historical and implied volatility calculations. Historical volatility was based on the standard deviation of daily total continuous returns, and implied volatility was based on the trailing month average of daily implied volatilities interpolating between the volatilities implied by stock call option contracts that were closest to the terms shown and closest to the money. The minimum volatility was based on a period of 1 year, 3 years and 2 years for the years ended December 31, 2018, 2017 and 2016, respectively. The maximum volatility was based on a period of 3 years, 1 year and 1 year for the years ended December 31, 2018, 2017 and 2016, respectively. The requisite service period is based on the criteria for the separate programs according to the vesting schedule. A summary of the status of the nonvested restricted shares as of December 31, 2018, and the changes for the year ended December 31, 2018, is presented below:
Nonvested Shares |  | Shares |  | Weighted Average Grant-Date Fair Value
Nonvested as of January 1, 2018 |  | 180,692 |  |  | $ | 81.13 |
Issued |  | 115,177 |  |  | 89.67 |
Vested |  | (106,434 | ) |  | 71.80 |
Forfeited |  | (1,658 | ) |  | 90.30 |
Nonvested as of December 31, 2018 |  | 187,777 |  |  | $ | 88.79 |
The total fair value of shares vested during the years ended December 31, 2018, 2017 and 2016 was $7.6 million, $10.5 million and $5.1 million, respectively. Stock Options Stock options are earned when the employee remains employed over the requisite service period and vest ratably over a period from 0.3 years to 2.3 years. Stock options exercised result in new common shares being issued on the open market by the Company. The fair value of stock option awards is determined using the Black-Scholes or Monte Carlo valuation models. No stock options were granted during the years ended December 31, 2018 or 2017. During the year ended December 31, 2016, 108,198 fully vested stock options were granted with a weighted average grant date fair value of $18.08 per option as a result of options exchanged in the merger with Post Properties. The following is a summary of the key assumptions used in the Monte Carlo valuation calculations for stock options granted during the year ended December 31, 2016:
|  | 2016
Risk free rate |  | 0.64% - 2.63%
Dividend yield |  | 3.81%
Volatility |  | 21.02% - 21.57%
Expected term |  | 1.11 - 2.11 years
The U.S. Treasury bill rate was used to represent the risk-free rate based on the expected life of the option. The current dividend yield at the time of grant was used to estimate the dividend yield over the life of the option. Volatility is based on the actual changes in the market value of MAA’s stock and is calculated using daily market value changes from the date of grant over a past period equal to the expected term of the stock options. The expected term represents an estimate of the period of time the stock options are expected to remain outstanding. A summary of the status of the stock options as of December 31, 2018 and the changes for the year ended December 31, 2018 is presented below:
Stock Options |  | Options |  | Weighted Average Exercise Price
Outstanding as of January 1, 2018 |  | 108,438 |  |  | $ | 72.93 |
Granted |  | — |  |  | — |
Exercised |  | (17,823 | ) |  | 51.42 |
Expired |  | — |  |  | — |
Outstanding as of December 31, 2018 |  | 90,615 |  |  | $ | 77.16 |
All options outstanding as of December 31, 2018 were exercisable and had an intrinsic value of $1.7 million with a weighted average remaining term of 5.9 years. There were 17,823 options and 21,006 options exercised during the years ended December 31, 2018 and 2017 respectively. Cash received from the exercise of stock options totaled $0.9 million and $0.4 million for the years ended December 31, 2018 and 2017, respectively. During the year ended December 31, 2016, no cash was received from the exercise of stock options as no options were exercised.

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- Definition The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -URI http://asc.fasb.org/subtopic&trid=2208855 + Details
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