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Share-based Compensation (Tables)
 12 Months Ended
Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of weighted average assumptions used to estimate the grant date fair value of options granted
The fair value of each stock option or LTIP Unit granted is estimated on the date of grant using the Black-Scholes option pricing model based on historical information with the following weighted average assumptions for grants during the years ended December 31, 2017, 2016, and 2015.
 
2017
 
2016
 
2015
Dividend yield
2.51
%
 
2.37
%
 
1.97
%
Expected life
4.4 years

 
3.7 years

 
3.6 years

Risk-free interest rate
1.73
%
 
1.57
%
 
1.43
%
Expected stock price volatility
28.10
%
 
26.76
%
 
32.34
%
Summary of the status of stock options and changes during the period
A summary of the status of the Company's stock options as of December 31, 2017, 2016, and 2015 and changes during the years ended December 31, 2017, 2016, and 2015 are as follows:
 
2017
 
2016
2015
 
Options Outstanding
 
Weighted Average
Exercise Price
 
Options Outstanding
 
Weighted Average
Exercise Price
Options
Outstanding
 
Weighted
Average
Exercise
Price
Balance at beginning of year
$
1,737,213

 
$
98.44

 
$
1,595,007

 
$
95.52

$
1,462,726

 
$
87.98

Granted
174,000

 
105.66

 
445,100

 
105.86

389,836

 
112.54

Exercised
(292,193
)
 
81.07

 
(192,875
)
 
76.90

(217,438
)
 
74.69

Lapsed or canceled
(70,301
)
 
121.68

 
(110,019
)
 
123.86

(40,117
)
 
98.61

Balance at end of year
$
1,548,719

 
$
101.48

 
$
1,737,213

 
$
98.44

$
1,595,007

 
$
95.52

Options exercisable at end of year
800,902

 
$
94.33

 
748,617

 
$
87.72

589,055

 
$
89.85

Weighted average fair value of options granted during the year
$
3,816,652

 
 

 
$
8,363,036

 
 

$
9,522,613

 
 
Schedule of other share-based compensation, activity
A summary of the Company's restricted stock as of December 31, 2017, 2016, and 2015 and charges during the years ended December 31, 2017, 2016, and 2015 are as follows:
 
2017
 
2016
 
2015
Balance at beginning of year
3,202,031

 
3,137,881

 
3,000,979

Granted
96,185

 
98,800

 
143,053

Canceled
—

 
(34,650
)
 
(6,151
)
Balance at end of year
3,298,216

 
3,202,031

 
3,137,881

Vested during the year
95,736

 
83,822

 
87,081

Compensation expense recorded
$
9,809,749

 
$
7,153,966

 
$
7,540,747

Weighted average fair value of restricted stock granted during the year
$
9,905,986

 
$
10,650,077

 
$
16,061,201