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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Share-Based Compensation Expense By Type [Table Text Block]
Share-based compensation expense was as follows:
 
Year Ended December 31,
 
2017
 
2016
 
2015
PUs
$
12,145

 
$
12,485

 
$
4,899

RSUs
9,183

 
7,846

 
6,032

Stock options
3,635

 
3,701

 
3,024

Total
$
24,963

 
$
24,032

 
$
13,955
Schedule of Share-based Compensation, Stock Options, Activity
Stock option transactions and related information for the year ended December 31, 2017 was as follows:
 
Options    
 
Weighted Average Exercise Price
 
Weighted Average Remaining Contractual Life (Years)
 
Aggregate Intrinsic Value
Outstanding as of January 1, 2017
748,371

 
$
53.24

 
 
 
 
Granted
109,431

 
$
107.49

 
 
 
 
Exercised
(273,026
)
 
$
46.46

 
 
 
 
Forfeited
(15,920
)
 
$
72.85

 
 
 
 
Outstanding as of December 31, 2017
568,856

 
$
67.14

 
7.6
 
$
31,490

Exercisable as of December 31, 2017
316,295

 
$
55.20

 
5.9
 
$
21,287
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions
The fair value of each option was estimated using the following assumptions:
 
2017
 
2016
 
2015
Expected volatility
27.38% - 27.47%

 
27.58% - 27.70%

 
27.95% - 28.00%

Dividend yield
0.00
%
 
0.00
%
 
0.00
%
Expected option life - years
6.0

 
6.0

 
6.0

Risk-free rate
1.9% - 2.1%

 
1.1% - 1.4%

 
1.5% - 1.7%
Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity
Restricted stock and restricted stock units transactions and related information for the year ended December 31, 2017 was as follows:
 
Restricted Stock and Restricted Units
 
Weighted Average Grant Date Fair Value
Non-vested as of January 1, 2017
371,559

 
$
63.98

Granted
118,562

 
$
107.57

Vested
(104,608
)
 
$
67.01

Forfeited
(31,466
)
 
$
71.29

Non-vested as of December 31, 2017
354,047

 
$
77.03
Schedule of Nonvested Performance-based Units Activity [Table Text Block]
A summary of activity for performance-vested restricted stock units transactions and related information for the year ended December 31, 2017 was as follows:
 
Performance Units
 
Weighted Average Grant Date Fair Value
Non-vested as of January 1, 2017
247,967

 
$
63.53

Granted
66,514

 
$
107.49

Vested
(60,286
)
 
$
66.32

Forfeited
(24,302
)
 
$
69.44

Non-vested as of December 31, 2017
229,893

 
$
74.90
Schedule of Share-based Payment Award, TSRs, Valuation Assumptions [Table Text Block]
Assumptions used to estimate the fair value of the TSR-based performance grant were as follows:
 
Year Ended December 31,
 
2017
 
2016
Expected volatility
26.00
%
 
27.60
%
Dividend yield
0.00
%
 
0.00
%
Expected life - years
1.57

 
3.00

Risk-free rate
1.83
%
 
0.87
%