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Goodwill and Other Intangible Assets - Textual (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Oct. 19, 2017
May 15, 2017
Goodwill [Line Items]        
Goodwill $ 845,687 $ 842,651    
Minimum        
Finite-Lived Intangible Assets [Line Items]        
Useful life 10 years      
Maximum        
Finite-Lived Intangible Assets [Line Items]        
Useful life 15 years      
Core Deposit Premium | Minimum        
Finite-Lived Intangible Assets [Line Items]        
Useful life 10 years      
Core Deposit Premium | Maximum        
Finite-Lived Intangible Assets [Line Items]        
Useful life 15 years      
OKSB Merger        
Goodwill [Line Items]        
Goodwill     $ 229,144  
OKSB Merger | Core Deposit Premium        
Finite-Lived Intangible Assets [Line Items]        
Finite-lived intangible assets acquired   42,100    
OKSB Merger | Other intangibles, net        
Finite-Lived Intangible Assets [Line Items]        
Finite-lived intangible assets acquired   3,800    
First Texas BHC, Inc.        
Goodwill [Line Items]        
Goodwill     $ 240,800  
First Texas BHC, Inc. | Core Deposit Premium        
Finite-Lived Intangible Assets [Line Items]        
Finite-lived intangible assets acquired   7,300    
Hardeman County Investment Company, Inc.        
Goodwill [Line Items]        
Goodwill       $ 29,371
Finite-Lived Intangible Assets [Line Items]        
Finite-lived intangible assets acquired   830    
Hardeman County Investment Company, Inc. | Core Deposit Premium        
Finite-Lived Intangible Assets [Line Items]        
Finite-lived intangible assets acquired   $ 7,800