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Equity Based Compensation Plans (Tables)
 12 Months Ended
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:
 202020192018
Stock price$244.74 $304.85 $262.09 
Risk-free interest rates0.83 %2.01 %2.76 %
Expected lives6 years6 years6 years
Volatility25.72 %19.56 %24.89 %
Dividend yield2.93 %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:
 202020192018
SharesWeighted-
average
exercise
price		SharesWeighted-
average
exercise
price		SharesWeighted-
average
exercise
price
Outstanding at beginning of year572,971 $251.10 612,954 $224.57 536,208 $211.41 
Granted149,020 244.74 148,147 304.85 119,361 262.09 
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 year613,109 255.86 572,971 251.10 612,954 224.57 
Options exercisable at year end361,985 245.83 305,379 223.90 322,837 206.63 
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:
 202020192018
SharesWeighted-
average
grant
price		SharesWeighted-
average
grant
price		SharesWeighted-
average
grant
price
Unvested at beginning of year114,877 $197.62 91,058 $180.99 90,823 $163.49 
Granted45,196 248.16 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 year132,603 214.34 114,877 197.62 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, 2017213,300 23,212 236,512 $75.03 7.5
Granted— — — 
Vested12,051 (12,051)— 
Converted(91,270)— (91,270)
Cancelled— — — 
Balance, December 31, 2018134,081 11,161 145,242 $75.03 6.5
Granted— — — 
Vested9,176 (9,176)— 
Converted— — — 
Cancelled— (95)(95)
Balance, December 31, 2019143,257 1,890 145,147 $75.03 5.2
Granted— — — 
Vested1,890 (1,890)— 
Converted(39,010)— (39,010)
Cancelled— — — 
Balance, December 31, 2020106,137 — 106,137 $84.47 3.6