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Note 11 - Equity-based Compensation (Tables)
3 Months Ended
Mar. 31, 2018
Notes Tables  
Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block]
   
 
 
 
 
Weighted Average
 
   
 
 
 
 
Grant Date
 
   
Restricted
   
Fair Value
 
   
Stock
   
Per Share
 
Outstanding as of December 31, 2017
   
51,290
    $
472.89
 
Granted
   
11,039
    $
707.20
 
Forfeited
   
(818
)   $
595.99
 
Vested
   
(24,171
)   $
391.19
 
Outstanding as of March 31, 2018
   
37,340
    $
592.42
 
                 
Vested and unissued as of March 31, 2018
   
3,185
    $
436.93
 
Schedule of Share-based Compensation, Stock Appreciation Rights Award Activity [Table Text Block]
   
Stock
Appreciation
Rights
   
Weighted
Average
Exercise
Price
   
Weighted
Average
Grant
Date
Fair
Value
   
Aggregate
Intrinsic
Value
(
in thousands
)
   
Weighted
Average
Remaining
Contractual
Term
(in
y
ears)
 
Outstanding as of December 31, 2017
   
102,458
    $
477.62
    $
100.91
    $
23,173
     
8.1
 
Granted
   
12,500
    $
707.17
    $
169.54
    $
-
     
9.8
 
Exercised
   
(11,157
)   $
432.23
    $
89.73
     
 
     
 
 
Forfeited
   
(2,249
)   $
422.31
    $
87.22
     
 
     
 
 
Outstanding as of March 31, 2018
   
101,552
    $
512.09
    $
110.89
    $
18,116
     
8.1
 
                                         
Vested and exercisable as of March 31, 2018
   
15,795
    $
482.94
    $
102.51
    $
3,225
     
7.8
 
Schedule of Share-based Payment Award, Other than Options, Valuation Assumptions [Table Text Block]
   
2018
 
Expected volatility
   
22.53
%
Risk-free interest rate
   
2.31
%
Expected term (in years)
   
6.25
 
Expected dividend yield
   
0.98
%