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Share and Unit-based Plans (Tables) | 12 Months Ended
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Dec. 31, 2020
Share-based Payment Arrangement [Abstract] |
Summary of Activity of Non-vested Stock Units | The following table summarizes the activity of non-vested stock units during the years ended December 31, 2020, 2019 and 2018:
| 2020 |  | 2019 |  | 2018
| 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 | 199,987 |  |  | $ | 43.59 |  |  | 129,457 |  |  | $ | 64.21 |  |  | 151,355 |  |  | $ | 73.32 |
Granted | 253,184 |  |  | 14.14 |  |  | 160,932 |  |  | 37.44 |  |  | 87,983 |  |  | 58.79 |
Vested | (140,224) |  |  | 39.53 |  |  | (85,157) |  |  | 62.84 |  |  | (108,991) |  |  | 74.04 |
Forfeited | (3,102) |  |  | 32.62 |  |  | (5,245) |  |  | 51.48 |  |  | (890) |  |  | 68.81 |
Balance at end of year | 309,845 |  |  | $ | 21.47 |  |  | 199,987 |  |  | $ | 43.59 |  |  | 129,457 |  |  | $ | 64.21 |
Summary of Activity of SARs Awards | The following table summarizes the activity of SARs awards during the years ended December 31, 2020, 2019 and 2018:
| 2020 |  | 2019 |  | 2018
| Units |  | Weighted Average Exercise Price |  | Units |  | Weighted Average Exercise Price |  | Units |  | Weighted Average Exercise Price
Balance at beginning of year | — |  |  | $ | — |  |  | — |  |  | $ | — |  |  | 235,439 |  |  | $ | 53.83 |
Exercised | — |  |  | — |  |  | — |  |  | — |  |  | (235,439) |  |  | 53.83 |
Balance at end of year | — |  |  | $ | — |  |  | — |  |  | $ | — |  |  | — |  |  | $ | — |
Schedule of LTIP Units Granted | The Company has granted the following LTIP units during the years ended December 31, 2020, 2019 and 2018:
Grant Date |  | Units |  | Type |  | Fair Value per LTIP Unit |  | Vest Date
1/1/2018 |  | 65,466 |  |  | Service-based |  | $ | 65.68 |  |  | 12/31/2020
1/1/2018 |  | 291,326 |  |  | Market-indexed |  | $ | 44.28 |  |  | 12/31/2020
1/29/2018 |  | 13,632 |  |  | Service-based |  | $ | 66.02 |  |  | 2/1/2022
1/29/2018 |  | 1,893 |  |  | Service-based |  | $ | 66.02 |  |  | 12/31/2020
1/29/2018 |  | 7,775 |  |  | Market-indexed |  | $ | 48.23 |  |  | 12/31/2020
3/2/2018 |  | 99,407 |  |  | Service-based |  | $ | 59.04 |  |  | 3/2/2018
4/26/2018 |  | 89,637 |  |  | Service-based |  | $ | 55.78 |  |  | 4/26/2018
|  | 569,136 |  |  |  |  |  |  |
1/1/2019 |  | 81,732 |  |  | Service-based |  | $ | 43.28 |  |  | 12/31/2021
1/1/2019 |  | 250,852 |  |  | Market-indexed |  | $ | 29.25 |  |  | 12/31/2021
9/1/2019 |  | 4,393 |  |  | Service-based |  | $ | 28.53 |  |  | 8/31/2022
9/1/2019 |  | 6,454 |  |  | Market-indexed |  | $ | 19.42 |  |  | 8/31/2022
|  | 343,431 |  |  |  |  |  |  |
1/1/2020 |  | 154,158 |  |  | Service-based |  | $ | 26.92 |  |  | 12/31/2022
1/1/2020 |  | 321,940 |  |  | Market-indexed |  | $ | 27.80 |  |  | 12/31/2022
3/1/2020 |  | 39,176 |  |  | Service-based |  | $ | 20.42 |  |  | 2/28/2023
3/1/2020 |  | 37,592 |  |  | Market-indexed |  | $ | 21.28 |  |  | 2/28/2023
|  | 552,866 |  |  |  |  |  |  |
Schedule LTIP Units Valuation Assumptions | The fair value of the market-indexed LTIP Units (Level 3) were estimated on the date of grant using a Monte Carlo Simulation model that based on the following assumptions:
Grant Date |  | Risk Free Interest Rate |  | Expected Volatility
1/1/2018 |  | 1.98 | % |  | 23.38 | %
1/29/2018 |  | 2.25 | % |  | 23.86 | %
1/1/2019 |  | 2.46 | % |  | 23.52 | %
9/1/2019 |  | 1.42 | % |  | 24.91 | %
1/1/2020 |  | 1.62 | % |  | 26.08 | %
3/1/2020 |  | 0.85 | % |  | 28.34 | %
Summary of Activity of Stock Options | The following table summarizes the activity of the non-vested LTIP Units during the years ended December 31, 2020, 2019 and 2018:
| 2020 |  | 2019 |  | 2018
| 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 | 616,219 |  |  | $ | 39.04 |  |  | 661,578 |  |  | $ | 48.38 |  |  | 636,632 |  |  | $ | 52.36 |
Granted | 552,866 |  |  | 26.59 |  |  | 343,431 |  |  | 32.40 |  |  | 569,136 |  |  | 51.78 |
Vested | (102,884) |  |  | 40.19 |  |  | (76,306) |  |  | 59.27 |  |  | (253,625) |  |  | 61.17 |
Forfeited | (282,149) |  |  | 44.28 |  |  | (312,484) |  |  | 46.55 |  |  | (290,565) |  |  | 52.58 |
Balance at end of year | 784,052 |  |  | $ | 28.11 |  |  | 616,219 |  |  | $ | 39.04 |  |  | 661,578 |  |  | $ | 48.38 |
The following table summarizes the activity of stock options for the years ended December 31, 2020, 2019 and 2018:
| 2020 |  | 2019 |  | 2018
| Options |  | Weighted Average Exercise Price |  | Options |  | Weighted Average Exercise Price |  | Options |  | Weighted Average Exercise Price
Balance at beginning of year | 35,565 |  |  | $ | 57.32 |  |  | 35,565 |  |  | $ | 57.32 |  |  | 35,565 |  |  | $ | 57.32 |
Granted(1) | 1,950 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Balance at end of year | 37,515 |  |  | $ | 54.34 |  |  | 35,565 |  |  | $ | 57.32 |  |  | 35,565 |  |  | $ | 57.32 |
(1)Pursuant to the terms of the Company's equity plan, the exercise price and number of options were adjusted so that the stock dividend paid on June 3, 2020 had no negative impact on the outstanding stock options (See Note 14–Stockholder's Equity).
Summary of Activity of Non-vested Phantom Stock Units | The following table summarizes the activity of the non-vested phantom stock units for the years ended December 31, 2020, 2019 and 2018:
| 2020 |  | 2019 |  | 2018
| 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 | 7,216 |  |  | $ | 43.29 |  |  | — |  |  | $ | — |  |  | 4,054 |  |  | $ | 79.82 |
Granted | 24,576 |  |  | 17.11 |  |  | 23,690 |  |  | 40.26 |  |  | 10,380 |  |  | 49.55 |
Vested | (27,130) |  |  | 20.94 |  |  | (16,474) |  |  | 38.94 |  |  | (12,193) |  |  | 54.40 |
Forfeited | — |  |  | — |  |  | — |  |  | — |  |  | (2,241) |  |  | 77.91 |
Balance at end of year | 4,662 |  |  | $ | 35.35 |  |  | 7,216 |  |  | $ | 43.29 |  |  | — |  |  | $ | — |
Compensation Cost Under the Share and Unit-based Plans | The following summarizes the compensation cost under the share and unit-based plans for the years ended December 31, 2020, 2019 and 2018:
| 2020 |  | 2019 |  | 2018
Stock units | $ | 4,159 |  |  | $ | 4,598 |  |  | $ | 6,355 |
LTIP units | 13,339 |  |  | 11,372 |  |  | 26,311 |
Stock options | — |  |  | 51 |  |  | 125 |
Phantom stock units | 568 |  |  | 702 |  |  | 760 |
| $ | 18,066 |  |  | $ | 16,723 |  |  | $ | 33,551 |

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