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Equity Based Compensation Plans (Tables)
 12 Months Ended
Dec. 31, 2019
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:

 
2019
 
2018
 
2017
Stock price
$
304.85

 
$
262.09

 
$
240.56

Risk-free interest rates
2.01
%
 
2.76
%
 
2.30
%
Expected lives
6 years

 
6 years

 
6 years

Volatility
19.56
%
 
24.89
%
 
24.10
%
Dividend yield
2.72
%
 
2.81
%
 
2.90
%
Summary of Stock Options Activity
A summary of the status of the Company’s stock option plans as of December 31, 2019, 2018, and 2017 and changes during the years ended on those dates is presented below:

 
2019
 
2018
 
2017
 
Shares
 
Weighted-
average
exercise
price
 
Shares
 
Weighted-
average
exercise
price
 
Shares
 
Weighted-
average
exercise
price
Outstanding at beginning of year
612,954

 
$
224.57

 
536,208

 
$
211.41

 
557,648

 
$
181.50

Granted
148,147

 
304.85

 
119,361

 
262.09

 
164,677

 
240.56

Exercised
(182,817
)
 
205.25

 
(39,175
)
 
159.05

 
(176,489
)
 
146.86

Forfeited and canceled
(5,313
)
 
257.87

 
(3,440
)
 
221.80

 
(9,628
)
 
160.40

Outstanding at end of year
572,971

 
251.10

 
612,954

 
224.57

 
536,208

 
211.41

Options exercisable at year end
305,379

 
223.90

 
322,837

 
206.63

 
223,796

 
191.09
Summary of Restricted Stock Activity
The following table summarizes information about restricted stock outstanding as of December 31, 2019, 2018 and 2017 and changes during the years ended:
 
2019
 
2018
 
2017
 
Shares
 
Weighted-
average
grant
price
 
Shares
 
Weighted-
average
grant
price
 
Shares
 
Weighted-
average
grant
price
Unvested at beginning of year
91,058

 
$
180.99

 
90,823

 
$
163.49

 
58,349

 
$
149.11

Granted
41,643

 
235.93

 
51,945

 
194.70

 
62,706

 
177.28

Vested
(13,222
)
 
143.56

 
(48,212
)
 
150.76

 
(29,675
)
 
170.17

Forfeited and canceled
(4,602
)
 
158.06

 
(3,498
)
 
158.71

 
(557
)
 
119.37

Unvested at end of year
114,877

 
197.62

 
91,058

 
180.99

 
90,823

 
163.49
Summary of Long Term Incentive Plan - Z Units
The following table summarizes information about the LTIP Units outstanding as of December 31, 2019:
 
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, 2016
181,027

 
60,027

 
241,054

 
$
75.11

 
8.5
Granted
—

 
—

 
—

 


 

Vested
32,961

 
(32,961
)
 
—

 


 

Converted
(688
)
 
—

 
(688
)
 


 

Cancelled
—

 
(3,854
)
 
(3,854
)
 


 

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