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Share and Unit-based Plans | 12 Months Ended
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Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Share and Unit-based Plans | Share and Unit-based Plans: The Company has established share and unit-based compensation plans for the purpose of attracting and retaining executive officers, directors and key employees. 2003 Equity Incentive Plan: The 2003 Equity Incentive Plan ("2003 Plan") authorizes the grant of stock awards, stock options, stock appreciation rights, stock units, stock bonuses, performance-based awards, dividend equivalent rights and OP Units or other convertible or exchangeable units. As of December 31, 2017, stock awards, stock units, LTIP Units (as defined below), stock appreciation rights ("SARs") and stock options have been granted under the 2003 Plan. All stock options or other rights to acquire common stock granted under the 2003 Plan have a term of 10 years or less. These awards were generally granted based on the performance of the Company and the employees. None of the awards have performance requirements other than a service condition of continued employment unless otherwise provided. All awards are subject to restrictions determined by the Company's compensation committee. The aggregate number of shares of common stock that may be issued under the 2003 Plan is 19,825,428 shares. As of December 31, 2017, there were 6,176,479 shares available for issuance under the 2003 Plan. Stock Awards: The value of the stock awards was determined by the market price of the Company's common stock on the date of the grant. The following table summarizes the activity of non-vested stock awards during the years ended December 31, 2017, 2016 and 2015:
| 2017 |  | 2016 |  | 2015
| Shares |  | Weighted Average Grant Date Fair Value |  | Shares |  | Weighted Average Grant Date Fair Value |  | Shares |  | Weighted Average Grant Date Fair Value
Balance at beginning of year | — |  |  | $ | — |  |  | 1,612 |  |  | $ | 62.01 |  |  | 9,189 |  |  | $ | 59.25 |
Granted | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Vested | — |  |  | — |  |  | (1,612 | ) |  | 62.01 |  |  | (7,577 | ) |  | 58.67 |
Balance at end of year | — |  |  | $ | — |  |  | — |  |  | $ | — |  |  | 1,612 |  |  | $ | 62.01 |
Stock Units: The stock units represent the right to receive upon vesting one share of the Company's common stock for one stock unit. The value of the stock units was determined by the market price of the Company's common stock on the date of the grant. The following table summarizes the activity of non-vested stock units during the years ended December 31, 2017, 2016 and 2015:
| 2017 |  | 2016 |  | 2015
| Units |  | Weighted Average Grant Date Fair Value |  | Units |  | Weighted Average Grant Date Fair Value |  | Units |  | Weighted Average Grant Date Fair Value
Balance at beginning of year | 148,428 |  |  | $ | 78.53 |  |  | 132,086 |  |  | $ | 74.58 |  |  | 144,374 |  |  | $ | 59.94 |
Granted | 86,827 |  |  | 66.46 |  |  | 85,601 |  |  | 79.22 |  |  | 77,282 |  |  | 86.53 |
Vested | (81,205 | ) |  | 75.62 |  |  | (69,259 | ) |  | 71.82 |  |  | (86,761 | ) |  | 61.29 |
Forfeited | (2,695 | ) |  | 69.57 |  |  | — |  |  | — |  |  | (2,809 | ) |  | 86.72 |
Balance at end of year | 151,355 |  |  | $ | 73.32 |  |  | 148,428 |  |  | $ | 78.53 |  |  | 132,086 |  |  | $ | 74.58 |
SARs: The executives and key employees have up to 10 years from the grant date to exercise the SARs. Upon exercise, the executives and key employees will receive unrestricted common shares for the appreciation in value of the SARs from the grant date to the exercise date. The Company determined the value of each SAR awarded during the year ended December 31, 2012 to be $9.67 using the Black‑Scholes Option Pricing Model based upon the following assumptions: volatility of 25.85%, dividend yield of 3.69%, risk free rate of 1.20%, current value of $59.57 and an expected term of 8 years. The value of each of the other outstanding SARs was determined at the grant date to be $7.68 based upon the following assumptions: volatility of 22.52%, dividend yield of 5.23%, risk free rate of 3.15%, current value of $61.17 and an expected term of 8 years. The assumptions for volatility and dividend yield were based on the Company's historical experience as a publicly traded company, the current value was based on the closing price on the date of grant and the risk free rate was based upon the interest rate of the 10-year Treasury bond on the date of grant. In connection with the payment of the Special Dividend (See Note 13—Stockholders' Equity), the compensation committee approved an adjustment to all outstanding SARs. The exercise price and number of outstanding SARs were adjusted such that each SAR had the same fair value to the holder before and after giving effect to the payment of the special dividend. As a result, the 407,823 outstanding SARs on December 8, 2015 with a weighted-average price of $56.49 were adjusted to 417,783 outstanding SARs with a weighted average price of $55.13 and the 417,783 outstanding SARs on January 6, 2016 with a weighted-average price of $55.13 were adjusted to 427,968 outstanding SARs with a weighted average price of $53.85. The following table summarizes the activity of SARs awards during the years ended December 31, 2017, 2016 and 2015:
| 2017 |  | 2016 |  | 2015
| Units |  | Weighted Average Exercise Price |  | Units |  | Weighted Average Exercise Price |  | Units |  | Weighted Average Exercise Price
Balance at beginning of year | 284,146 |  |  | $ | 53.85 |  |  | 417,783 |  |  | $ | 55.13 |  |  | 772,639 |  |  | $ | 56.67 |
Granted | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Exercised | (48,707 | ) |  | 53.95 |  |  | (143,822 | ) |  | 53.73 |  |  | (364,807 | ) |  | 56.86 |
Special dividend adjustment | — |  |  | — |  |  | 10,185 |  |  | 53.88 |  |  | 9,951 |  |  | 55.13 |
Balance at end of year | 235,439 |  |  | $ | 53.83 |  |  | 284,146 |  |  | $ | 53.85 |  |  | 417,783 |  |  | $ | 55.13 |
Long-Term Incentive Plan Units: Under the Long-Term Incentive Plan ("LTIP"), each award recipient is issued a form of operating partnership units ("LTIP Units") in the Operating Partnership. Upon the occurrence of specified events and subject to the satisfaction of applicable vesting conditions, LTIP Units (after conversion into OP Units) are ultimately redeemable for common stock of the Company, or cash at the Company's option, on a one-unit for one-share basis. LTIP Units receive cash dividends based on the dividend amount paid on the common stock of the Company. The LTIP may include both market-indexed awards and service-based awards. The market-indexed LTIP Units vest over the service period of the award based on the percentile ranking of the Company in terms of total return to stockholders (the "Total Return") per common stock share relative to the Total Return of a group of peer REITs, as measured at the end of the measurement period. The fair value of the service-based LTIP Units was determined by the market price of the Company's common stock on the date of the grant. The fair value of the market-indexed LTIP Units are estimated on the date of grant using a Monte Carlo Simulation model. The stock price of the Company, along with the stock prices of the group of peer REITs (for market-indexed awards), is assumed to follow the Multivariate Geometric Brownian Motion Process. Multivariate Geometric Brownian Motion is a common assumption when modeling in financial markets, as it allows the modeled quantity (in this case, the stock price) to vary randomly from its current value and take any value greater than zero. The volatilities of the returns on the share price of the Company and the peer group REITs were estimated based on a look-back period. The expected growth rate of the stock prices over the "derived service period" is determined with consideration of the risk free rate as of the grant date. On January 1, 2015, the Company granted 49,451 LTIP Units with a grant date fair value of $83.41 per LTIP Unit that vested in equal annual installments over a service period that ended December 31, 2017. Concurrently, the Company granted 186,450 market-indexed LTIP Units ("2015 Market LTIP Units") at a grant date fair value of $66.37 per LTIP Unit that vested over a service period ending December 31, 2015. The 2015 Market LTIP Units were equally divided between two types of awards. The terms of both types of awards were the same, except one award has an additional 3% absolute Total Return requirement, which if it is not met, then such LTIP Units would not have vested. The grant date fair value of the 2015 Market LTIP Units assumed a risk free interest rate of 0.25% and an expected volatility of 16.81%. On January 7, 2016, the compensation committee determined that the 2015 Market LTIP Units had vested at a 130% level, based on the Company's percentile ranking in terms of Total Return per common stock share compared to the Total Return of a group of peer REITs during the period of January 1, 2015 to December 31, 2015. In addition, the compensation committee determined that the applicable 3% absolute Total Return requirement was exceeded. As a result, an additional 55,934 fully-vested LTIP Units were granted on December 31, 2015. On March 6, 2015, the Company granted 132,607 LTIP Units at a fair value of $86.72 per LTIP Unit that were fully vested on the grant date. On January 1, 2016, the Company granted 58,786 LTIP Units with a grant date fair value of $80.69 per LTIP Unit that will vest in equal annual installments over a service period ending December 31, 2018. Concurrently, the Company granted 266,899 market-indexed LTIP Units ("2016 Market LTIP Units") at a grant date fair value of $53.32 per LTIP Unit that vest over a service period ending December 31, 2018. The fair value of the 2016 Market LTIP Units was estimated on the date of grant using a Monte Carlo Simulation model that assumed a risk free interest rate of 1.32% and an expected volatility of 20.31%. On March 4, 2016, the Company granted 154,686 LTIP Units at a fair value of $79.20 per LTIP Unit that were fully vested on the grant date. On January 1, 2017, the Company granted 66,079 LTIP Units with a grant date fair value of $70.84 per LTIP Unit that will vest in equal annual installments over a service period ending December 31, 2019. Concurrently, the Company granted 297,849 market-indexed LTIP Units ("2017 Market LTIP Units") at a grant date fair value of $47.15 per LTIP Unit that will vest over a service period ending December 31, 2019. The fair value of the 2017 Market LTIP Units was estimated on the date of grant using a Monte Carlo Simulation model that assumed a risk free interest rate of 1.49% and an expected volatility of 20.75%. On March 3, 2017, the Company granted 134,742 LTIP Units at a fair value of $66.57 per LTIP Unit that were fully vested on the grant date. On June 1, 2017, the Company granted 1,522 LTIP Units with a grant date fair value of $58.31 per LTIP Unit that will vest in equal annual installments over a service period ending May 29, 2020. Concurrently, the Company granted 6,714 market-indexed LTIP Units at a grant date fair value of $39.66 per LTIP Unit that will vest over a service period ending May 29, 2020. The fair value of the market-indexed LTIP Units was estimated on the date of grant using a Monte Carlo Simulation model that assumed a risk free interest rate of 1.45% and an expected volatility of 21.40%. The following table summarizes the activity of the non-vested LTIP Units during the years ended December 31, 2017, 2016 and 2015:
| 2017 |  | 2016 |  | 2015
| Units |  | Weighted Average Grant Date Fair Value |  | Units |  | Weighted Average Grant Date Fair Value |  | Units |  | Weighted Average Grant Date Fair Value
Balance at beginning of year | 322,572 |  |  | $ | 58.18 |  |  | 56,315 |  |  | $ | 73.24 |  |  | 46,695 |  |  | $ | 58.89 |
Granted | 506,906 |  |  | 55.33 |  |  | 480,371 |  |  | 65.00 |  |  | 424,442 |  |  | 74.71 |
Vested | (192,846 | ) |  | 69.93 |  |  | (214,114 | ) |  | 77.45 |  |  | (414,822 | ) |  | 73.13 |
Forfeited | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  |  |
Balance at end of year | 636,632 |  |  | $ | 52.36 |  |  | 322,572 |  |  | $ | 58.18 |  |  | 56,315 |  |  | $ | 73.24 |
Stock Options: The Company measured the value of each option awarded during the year ended December 31, 2012 to be $9.67 using the Black-Scholes Option Pricing Model based upon the following assumptions: volatility of 25.85%, dividend yield of 3.69%, risk free rate of 1.20%, current value of $59.57 and an expected term of 8 years. The assumptions for volatility and dividend yield were based on the Company's historical experience as a publicly traded company, the current value was based on the closing price on the date of grant and the risk free rate was based upon the interest rate of the 10-year Treasury bond on the date of grant. In connection with the payment of the Special Dividend (See Note 13—Stockholders' Equity), the compensation committee approved an adjustment to all outstanding stock options. The exercise price and number of outstanding stock options were adjusted such that each stock option had the same fair value to the holder before and after giving effect to the payment of the Special Dividend. As a result, the 10,068 outstanding stock options on December 8, 2015 with a weighted-average price of $59.57 were adjusted to 10,314 outstanding stock options with a weighted average price of $58.15 and the 10,314 outstanding stock options on January 6, 2016 with a weighted-average price of $58.15 were adjusted to 10,565 outstanding stock options with a weighted average price of $56.77. On May 30, 2017, the Company granted 25,000 non-qualified stock options with a grant date fair value of $10.02 that will vest on May 30, 2019. The Company measured the value of each option awarded using the Black-Scholes Option Pricing Model based upon the following assumptions: volatility of 30.19%, dividend yield of 4.93%, risk free rate of 2.08%, current value of $57.55 and an expected term of 8 years. The following table summarizes the activity of stock options for the years ended December 31, 2017, 2016 and 2015:
| 2017 |  | 2016 |  | 2015
| Options |  | Weighted Average Exercise Price |  | Options |  | Weighted Average Exercise Price |  | Options |  | Weighted Average Exercise Price
Balance at beginning of year | 10,565 |  |  | $ | 56.77 |  |  | 10,314 |  |  | $ | 58.15 |  |  | 10,068 |  |  | $ | 59.57 |
Granted | 25,000 |  |  | 57.55 |  |  | — |  |  | — |  |  | — |  |  | — |
Exercised | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Special dividend adjustment | — |  |  | — |  |  | 251 |  |  | 56.77 |  |  | 246 |  |  | 58.15 |
Balance at end of year | 35,565 |  |  | $ | 57.32 |  |  | 10,565 |  |  | $ | 56.77 |  |  | 10,314 |  |  | $ | 58.15 |
Directors' Phantom Stock Plan: The Directors' Phantom Stock Plan offers non-employee members of the board of directors ("Directors") the opportunity to defer their cash compensation and to receive that compensation in common stock rather than in cash after termination of service or a predetermined period. Compensation generally includes the annual retainers payable by the Company to the Directors. Deferred amounts are generally credited as units of phantom stock at the beginning of each three-year deferral period by dividing the present value of the deferred compensation by the average fair market value of the Company's common stock at the date of award. Compensation expense related to the phantom stock awards was determined by the amortization of the value of the stock units on a straight-line basis over the applicable service period. The stock units (including dividend equivalents) vest as the Directors' services (to which the fees relate) are rendered. Vested phantom stock units are ultimately paid out in common stock on a one-unit for one-share basis. To the extent elected by a Director, stock units receive dividend equivalents in the form of additional stock units based on the dividend amount paid on the common stock. The aggregate number of phantom stock units that may be granted under the Directors' Phantom Stock Plan is 500,000. As of December 31, 2017, there were 169,755 stock units available for grant under the Directors' Phantom Stock Plan. The following table summarizes the activity of the non-vested phantom stock units for the years ended December 31, 2017, 2016 and 2015:
| 2017 |  | 2016 |  | 2015
| Stock Units |  | Weighted Average Grant Date Fair Value |  | Stock Units |  | Weighted Average Grant Date Fair Value |  | Stock Units |  | Weighted Average Grant Date Fair Value
Balance at beginning of year | 5,845 |  |  | $ | 81.47 |  |  | — |  |  | $ | — |  |  | 9,269 |  |  | $ | 58.35 |
Granted | 8,760 |  |  | 68.93 |  |  | 21,088 |  |  | 80.21 |  |  | 13,351 |  |  | 78.72 |
Vested | (10,551 | ) |  | 71.69 |  |  | (15,243 | ) |  | 79.73 |  |  | (20,162 | ) |  | 72.17 |
Forfeited | — |  |  | — |  |  | — |  |  | — |  |  | (2,458 | ) |  | 55.62 |
Balance at end of year | 4,054 |  |  | $ | 79.82 |  |  | 5,845 |  |  | $ | 81.47 |  |  | — |  |  | $ | — |
Employee Stock Purchase Plan ("ESPP"): The ESPP authorizes eligible employees to purchase the Company's common stock through voluntary payroll deductions made during periodic offering periods. Under the ESPP common stock is purchased at a 15% discount from the lesser of the fair value of common stock at the beginning and end of the offering period. A maximum of 750,000 shares of common stock is available for purchase under the ESPP. The number of shares available for future purchase under the plan at December 31, 2017 was 450,306. Compensation: The following summarizes the compensation cost under the share and unit-based plans for the years ended December 31, 2017, 2016 and 2015:
| 2017 |  | 2016 |  | 2015
Stock awards | $ | — |  |  | $ | 20 |  |  | $ | 252 |
Stock units | 6,045 |  |  | 6,305 |  |  | 6,041 |
LTIP units | 30,161 |  |  | 32,957 |  |  | 26,622 |
Stock options | 85 |  |  | 16 |  |  | 16 |
Phantom stock units | 714 |  |  | 1,231 |  |  | 1,444 |
| $ | 37,005 |  |  | $ | 40,529 |  |  | $ | 34,375 |
The Company capitalized share and unit-based compensation costs of $6,206, $7,241 and $6,008 for the years ended December 31, 2017, 2016 and 2015, respectively. The fair value of the stock awards and stock units that vested during the years ended December 31, 2017, 2016 and 2015 was $5,257, $5,644 and $8,794, respectively. Unrecognized compensation costs of share and unit-based plans at December 31, 2017 consisted of $286 from LTIP Units, $3,239 from stock units, $176 from stock options and $324 from phantom stock units.

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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -URI http://asc.fasb.org/subtopic&trid=2208855 + Details
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