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Equity Based Compensation Plans (Tables) | 12 Months Ended
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Dec. 31, 2020
Share-based Payment Arrangement, Noncash Expense [Abstract] |
Weighted Average Assumptions Used to Estimate Fair Value of Stock Options | The fair value of stock options was estimated on the date of grant using the Black-Scholes option pricing model with the following weighted average assumptions used for grants:
| 2020 |  | 2019 |  | 2018
Stock price | $ | 244.74 |  |  | $ | 304.85 |  |  | $ | 262.09 |
Risk-free interest rates | 0.83 | % |  | 2.01 | % |  | 2.76 | %
Expected lives | 6 years |  | 6 years |  | 6 years
Volatility | 25.72 | % |  | 19.56 | % |  | 24.89 | %
Dividend yield | 2.93 | % |  | 2.72 | % |  | 2.81 | %
Summary of Stock Options Activity | A summary of the status of the Company’s stock option plans as of December 31, 2020, 2019, and 2018 and changes during the years ended on those dates is presented below:
| 2020 |  | 2019 |  | 2018
| Shares |  | Weighted- average exercise price |  | Shares |  | Weighted- average exercise price |  | Shares |  | Weighted- average exercise price
Outstanding at beginning of year | 572,971 |  |  | $ | 251.10 |  |  | 612,954 |  |  | $ | 224.57 |  |  | 536,208 |  |  | $ | 211.41 |
Granted | 149,020 |  |  | 244.74 |  |  | 148,147 |  |  | 304.85 |  |  | 119,361 |  |  | 262.09 |
Exercised | (70,802) |  |  | 208.57 |  |  | (182,817) |  |  | 205.25 |  |  | (39,175) |  |  | 159.05 |
Forfeited and canceled | (38,080) |  |  | 228.64 |  |  | (5,313) |  |  | 257.87 |  |  | (3,440) |  |  | 221.80 |
Outstanding at end of year | 613,109 |  |  | 255.86 |  |  | 572,971 |  |  | 251.10 |  |  | 612,954 |  |  | 224.57 |
Options exercisable at year end | 361,985 |  |  | 245.83 |  |  | 305,379 |  |  | 223.90 |  |  | 322,837 |  |  | 206.63 |
Summary of Restricted Stock Activity | The following table summarizes information about restricted stock outstanding as of December 31, 2020, 2019 and 2018 and changes during the years ended:
| 2020 |  | 2019 |  | 2018
| Shares |  | Weighted- average grant price |  | Shares |  | Weighted- average grant price |  | Shares |  | Weighted- average grant price
Unvested at beginning of year | 114,877 |  |  | $ | 197.62 |  |  | 91,058 |  |  | $ | 180.99 |  |  | 90,823 |  |  | $ | 163.49 |
Granted | 45,196 |  |  | 248.16 |  |  | 41,643 |  |  | 235.93 |  |  | 51,945 |  |  | 194.70 |
Vested | (15,116) |  |  | 170.61 |  |  | (13,222) |  |  | 143.56 |  |  | (48,212) |  |  | 150.76 |
Forfeited and canceled | (12,354) |  |  | 184.11 |  |  | (4,602) |  |  | 158.06 |  |  | (3,498) |  |  | 158.71 |
Unvested at end of year | 132,603 |  |  | 214.34 |  |  | 114,877 |  |  | 197.62 |  |  | 91,058 |  |  | 180.99 |
Summary of Long Term Incentive Plan - Z Units | The following table summarizes information about the LTIP Units outstanding as of December 31, 2020:
| Long-Term Incentive Plan - LTIP Units
| Total Vested Units |  | Total Unvested Units |  | Total Outstanding Units |  | Weighted- average Grant-date Fair Value |  | Weighted- average Remaining Contractual Life (years)
Balance, December 31, 2017 | 213,300 |  |  | 23,212 |  |  | 236,512 |  |  | $ | 75.03 |  |  | 7.5
Granted | — |  |  | — |  |  | — |  |  |  |  |
Vested | 12,051 |  |  | (12,051) |  |  | — |  |  |  |  |
Converted | (91,270) |  |  | — |  |  | (91,270) |  |  |  |  |
Cancelled | — |  |  | — |  |  | — |  |  |  |  |
Balance, December 31, 2018 | 134,081 |  |  | 11,161 |  |  | 145,242 |  |  | $ | 75.03 |  |  | 6.5
Granted | — |  |  | — |  |  | — |  |  |  |  |
Vested | 9,176 |  |  | (9,176) |  |  | — |  |  |  |  |
Converted | — |  |  | — |  |  | — |  |  |  |  |
Cancelled | — |  |  | (95) |  |  | (95) |  |  |  |  |
Balance, December 31, 2019 | 143,257 |  |  | 1,890 |  |  | 145,147 |  |  | $ | 75.03 |  |  | 5.2
Granted | — |  |  | — |  |  | — |  |  |  |  |
Vested | 1,890 |  |  | (1,890) |  |  | — |  |  |  |  |
Converted | (39,010) |  |  | — |  |  | (39,010) |  |  |  |  |
Cancelled | — |  |  | — |  |  | — |  |  |  |  |
Balance, December 31, 2020 | 106,137 |  |  | — |  |  | 106,137 |  |  | $ | 84.47 |  |  | 3.6

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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Disclosure of the number and weighted-average grant date fair value for restricted stock and restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock and restricted stock units that were granted, vested, or forfeited during the year. + References No definition available. + Details
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