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Equity Based Compensation Plans (Tables)
 12 Months Ended
Dec. 31, 2017
Share-based Compensation [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:

 
2017
 
2016
 
2015
Stock price
$
240.56

 
$
219.60

 
$
227.75

Risk-free interest rates
2.30
%
 
2.08
%
 
1.83
%
Expected lives
6 years

 
6 years

 
6 years

Volatility
24.10
%
 
26.47
%
 
20.06
%
Dividend yield
2.90
%
 
2.89
%
 
2.73
%
Summary of stock options activity
A summary of the status of the Company’s stock option plans as of December 31, 2017, 2016, and 2015 and changes during the years ended on those dates is presented below:

 
2017
 
2016
 
2015
 
Shares
 
Weighted-
average
exercise
price
 
Shares
 
Weighted-
average
exercise
price
 
Shares
 
Weighted-
average
exercise
price
Outstanding at beginning of year
557,648

 
$
181.50

 
525,094

 
$
154.98

 
664,785

 
$
138.78

Granted
164,677

 
240.56

 
207,429

 
219.60

 
78,600

 
227.75

Exercised
(176,489
)
 
146.86

 
(138,054
)
 
138.79

 
(203,556
)
 
131.53

Forfeited and canceled
(9,628
)
 
160.40

 
(36,821
)
 
178.18

 
(14,735
)
 
136.11

Outstanding at end of year
536,208

 
211.41

 
557,648

 
181.50

 
525,094

 
154.98

Options exercisable at year end
223,796

 
191.09

 
290,340

 
160.90

 
342,048

 
152.42
Restricted stock activity
The following table summarizes information about restricted stock outstanding as of December 31, 2017, 2016 and 2015 and changes during the years ended:
 
2017
 
2016
 
2015
 
Shares
 
Weighted-
average
grant
price
 
Shares
 
Weighted-
average
grant
price
 
Shares
 
Weighted-
average
grant
price
Unvested at beginning of year
58,349

 
$
149.11

 
54,676

 
$
147.10

 
25,820

 
$
168.22

Granted
62,706

 
177.28

 
49,183

 
150.13

 
56,177

 
155.21

Vested
(29,675
)
 
170.17

 
(38,427
)
 
147.12

 
(22,939
)
 
148.20

Forfeited and canceled
(557
)
 
119.37

 
(7,083
)
 
141.76

 
(4,382
)
 
122.06

Unvested at end of year
90,823

 
163.49

 
58,349

 
149.11

 
54,676

 
147.10
Summary of long term incentive plan - Z Units
The following table summarizes information about the LTIP Units outstanding as of December 31, 2017:
 
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, 2014
181,919

 
151,067

 
332,986

 
$
71.14

 
10.5
Granted
—

 
—

 
—

 


 

Vested
36,650

 
(36,650
)
 
—

 


 

Converted
(74,384
)
 
—

 
(74,384
)
 


 

Cancelled
—

 
(8,260
)
 
(8,260
)
 


 

Balance, December 31, 2015
144,185

 
106,157

 
250,342

 
$
75.41

 
9.5
Granted
—

 
—

 
—

 


 

Vested
36,842

 
(36,842
)
 
—

 


 

Converted
—

 
—

 
—

 


 

Cancelled
—

 
(9,288
)
 
(9,288
)
 


 

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