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Goodwill and Other Intangible Assets - Textual (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Oct. 19, 2017
Oct. 18, 2017
May 15, 2017
May 14, 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 $ 13,545    
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 $ 11,485
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