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Investment Securities - Summary of Amortized Cost and Approximate Fair Value of Investment Securities (Detail) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 680,637 $ 692,674
Gross Unrealized Gains 10,597 920
Gross Unrealized Losses (1,405) (30,300)
Fair Value 689,829 663,294
Equity securities, at fair value   1,718
Investment securities, at fair value 708,359 665,012
Agency-guaranteed residential mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 299,370 311,267
Gross Unrealized Gains 2,023 0
Gross Unrealized Losses (2) (5,893)
Fair Value 301,391 305,374
Corporate notes    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 381,267 381,407
Gross Unrealized Gains 8,574 920
Gross Unrealized Losses (1,403) (24,407)
Fair Value 388,438 $ 357,920
Equity securities    
Debt Securities, Available-for-sale [Line Items]    
Equity securities, at fair value 1,373  
Interest-only classes of agency-guaranteed home equity conversion mortgage-backed securities, reported at fair value based on a fair value option election    
Debt Securities, Available-for-sale [Line Items]    
Interest-only classes of agency-guaranteed home equity conversion mortgage-backed securities $ 17,157