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Investment Securities - Summary of Amortized Cost and Approximate Fair Value of Investment Securities (Detail) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 1,174,188 $ 557,891
Gross Unrealized Gains 32,890 19,307
Gross Unrealized Losses (647) 0
Fair Value 1,206,431 577,198
Equity securities 3,854 2,406
Total investment securities, at fair value 1,210,285 595,876
Asset-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 372,640  
Gross Unrealized Gains 4,515  
Gross Unrealized Losses (10)  
Fair Value 377,145  
U.S. government agency securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 20,000  
Gross Unrealized Gains 34  
Gross Unrealized Losses 0  
Fair Value 20,034  
Agency-guaranteed mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 61,178 273,252
Gross Unrealized Gains 1,913 5,069
Gross Unrealized Losses 0 0
Fair Value 63,091 278,321
Agency-guaranteed collateralized mortgage obligations    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 160,950  
Gross Unrealized Gains 916  
Gross Unrealized Losses (99)  
Fair Value 161,767  
Collateralized loan obligations    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 32,367  
Gross Unrealized Gains 0  
Gross Unrealized Losses 0  
Fair Value 32,367  
Corporate notes    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 372,764 284,639
Gross Unrealized Gains 24,144 14,238
Gross Unrealized Losses (164) 0
Fair Value 396,744 298,877
Private label collateralized mortgage obligations    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 136,943  
Gross Unrealized Gains 423  
Gross Unrealized Losses (374)  
Fair Value 136,992  
State and political subdivision debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 17,346  
Gross Unrealized Gains 945  
Gross Unrealized Losses 0  
Fair Value $ 18,291  
Interest-only GNMA securities    
Debt Securities, Available-for-sale [Line Items]    
Interest-only GNMA securities   $ 16,272