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Note 7 - Goodwill and Other Intangible Assets (Details Textual) - USD ($)
3 Months Ended 12 Months Ended
Sep. 30, 2017
Sep. 30, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Oct. 19, 2017
May 15, 2017
Sep. 09, 2016
Oct. 29, 2015
Feb. 27, 2015
Goodwill     $ 842,651,000 $ 348,505,000            
Goodwill, Written off Related to Sale of Business Unit $ 4,100,000                  
Finite-Lived Intangible Assets, Period Increase (Decrease) $ (1,300,000)                  
Amortization of Intangible Assets     $ 7,668,000 5,945,000 $ 4,889,000          
Minimum [Member]                    
Finite-Lived Intangible Asset, Useful Life     10 years              
Maximum [Member]                    
Finite-Lived Intangible Asset, Useful Life     15 years              
Core Deposit Premium [Member]                    
Amortization of Intangible Assets     $ 6,000,000 4,400,000 3,900,000          
Core Deposit Premium [Member] | Salina Banking Operations [Member]                    
Decrease in Intangible Assets from The Sale of Business   $ 382,000                
Core Deposit Premium [Member] | Minimum [Member]                    
Finite-Lived Intangible Asset, Useful Life     10 years              
Core Deposit Premium [Member] | Maximum [Member]                    
Finite-Lived Intangible Asset, Useful Life     15 years              
Books of Business Intangible [Member]                    
Amortization of Intangible Assets     $ 1,100,000 1,100,000 528,000          
Purchased Credit Card Relationships [Member]                    
Amortization of Intangible Assets     414,000 414,000 414,000          
OKSB Merger [Member]                    
Goodwill           $ 228,932,000        
OKSB Merger [Member] | Core Deposit Premium [Member]                    
Finite-lived Intangible Assets Acquired     42,100,000              
OKSB Merger [Member] | Other Intangible Assets [Member]                    
Finite-lived Intangible Assets Acquired     3,800,000              
First Texas BHC, Inc. [Member]                    
Goodwill           $ 238,000,000        
First Texas BHC, Inc. [Member] | Core Deposit Premium [Member]                    
Finite-lived Intangible Assets Acquired     7,300,000              
Hardeman County Investment Company, Inc. [Member]                    
Goodwill             $ 29,371,000      
Finite-lived Intangible Assets Acquired     830,000              
Hardeman County Investment Company, Inc. [Member] | Core Deposit Premium [Member]                    
Finite-lived Intangible Assets Acquired     $ 7,800,000              
Citizens National Bank [Member]                    
Goodwill               $ 23,376,000    
Finite-lived Intangible Assets Acquired       591,000            
Citizens National Bank [Member] | Core Deposit Premium [Member]                    
Finite-lived Intangible Assets Acquired       $ 5,100,000            
Liberty Bancshares, Inc. [Member]                    
Goodwill                   $ 95,207,000
Liberty Bancshares, Inc. [Member] | Core Deposit Premium [Member]                    
Finite-lived Intangible Assets Acquired         14,600,000          
Community First Bancshares Inc [Member]                    
Goodwill                   $ 110,440,000
Community First Bancshares Inc [Member] | Core Deposit Premium [Member]                    
Finite-lived Intangible Assets Acquired         11,300,000          
Community First Bancshares Inc [Member] | Books of Business Intangible [Member]                    
Finite-lived Intangible Assets Acquired         420,000          
Ozark Trust [Member]                    
Goodwill                 $ 13,300,000  
Ozark Trust [Member] | Books of Business Intangible [Member]                    
Finite-lived Intangible Assets Acquired         $ 9,700,000