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Investment Securities (Details Textual) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Oct. 20, 2017
Debt Securities, Available-for-sale [Line Items]        
Available For Sale Securities Pledged As Collateral Amortized Cost $ 486,200      
Proceeds from sales of securities available-for-sale 1,584,860 $ 571,800 $ 759,300  
Federal Home Loan Bank stock, at cost 40,924 32,103    
Purchases of FHLB Stock 159,000      
Redemptions of FHLB stock (150,100)      
Available-for-sale Securities, Gross Realized Gains 29,584 9,225 $ 3,295  
Federal Home Loan Bank of San Francisco        
Debt Securities, Available-for-sale [Line Items]        
Increase (Decrease) in Marketable Securities, Restricted 8,800      
CU Bancorp        
Debt Securities, Available-for-sale [Line Items]        
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Marketable Securities       $ 446,980
CU Bancorp        
Debt Securities, Available-for-sale [Line Items]        
Available-for-sale Securities, Gross Realized Gains $ 0      
CU Bancorp | Available-for-sale Securities        
Debt Securities, Available-for-sale [Line Items]        
Proceeds from sales of securities available-for-sale   $ 404,500