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Note 11 - Equity-based Compensation - Stock Appreciation Rights (Details) - Stock Appreciation Rights (SARs) [Member] - USD ($)
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Outstanding, stock appreciation rights (in shares) 102,458  
Outstanding, weighted average exercise price (in dollars per share) $ 477.62  
Outstanding, beginning balance, weighted average fair value (in dollars per share) $ 100.91  
Outstanding, aggregate intrinsic value $ 18,116 $ 23,173
Weighted average remaining contractual term, outstanding (Year) 8 years 36 days 8 years 36 days
Granted, stock appreciation rights (in shares) 12,500  
Granted, weighted average exercise price (in dollars per share) $ 707.17  
Granted, weighted average fair value (in dollars per share) $ 169.54  
Weighted average remaining contractual term, granted (Year) 9 years 292 days  
Exercised, stock appreciation rights (in shares) 11,157  
Exercised, weighted average exercise price (in dollars per share) $ 432.23  
Exercised, weighted average fair value (in dollars per share) $ 89.73  
Forfeited, stock appreciation rights (in shares) (2,249)  
Forfeited, weighted average exercise price (in dollars per share) $ 422.31  
Forfeited, weighted average fair value (in dollars per share) $ 87.22  
Outstanding, stock appreciation rights (in shares) 101,552 102,458
Outstanding, weighted average exercise price (in dollars per share) $ 512.09 $ 477.62
Outstanding, beginning balance, weighted average fair value (in dollars per share) $ 110.89 $ 100.91
Vested and exercisable, stock appreciation rights (in shares) 15,795  
Vested and exercisable, weighted average exercise price (in dollars per share) $ 482.94  
Vested and exercisable, weighted average fair value (in dollars per share) $ 102.51  
Vested and exercisable, aggregate intrinsic value $ 3,225  
Weighted average remaining contractual term, vested and exercisable (Year) 7 years 292 days