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Investment Securities Narrative (Details) - USD ($)
$ in Thousands
Aug. 01, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Debt Securities, Available-For-Sale, Transfer To Held-To-Maturity, Amortized Cost $ 665,500      
Debt Securities, Available-For-Sale, Transfer To Held-To-Maturity, Fair Value 641,700      
Amortized cost basis   $ 2,115,265    
Estimated Fair Value   2,080,294 $ 2,547,956  
Residential mortgage-backed securities        
Debt Securities, Available-For-Sale, Transfer To Held-To-Maturity, Amortized Cost 505,500      
Debt Securities, Available-For-Sale, Transfer To Held-To-Maturity, Fair Value 485,300      
Amortized cost basis   476,973    
Estimated Fair Value   463,344    
Auction rate securities        
Amortized cost basis   107,410    
Estimated Fair Value   102,994    
State and municipal securities        
Debt Securities, Available-For-Sale, Transfer To Held-To-Maturity, Amortized Cost 160,000      
Debt Securities, Available-For-Sale, Transfer To Held-To-Maturity, Fair Value $ 156,400      
Amortized cost basis   282,383    
Estimated Fair Value   279,095    
Pooled trust preferred securities        
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held   11,500 11,500 $ 11,500
Collateral Pledged        
Available-for-sale securities pledged as collateral   $ 1,000,000 $ 1,800,000