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Investment Securities - Summary of Amortized Cost and Approximate Fair Value of Investment Securities (Detail) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 698,260 $ 472,300
Gross Unrealized Gains 347 2,639
Gross Unrealized Losses (31,575) (3,568)
Available-for-sale securities, debt securities 667,032 471,371
Investment securities, at fair value 668,851 471,371
Investment securities, at fair value   471,371
Corporate notes    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 381,475 44,959
Gross Unrealized Gains 347 1,130
Gross Unrealized Losses (20,208) 0
Available-for-sale securities, debt securities 361,614 46,089
Agency-guaranteed residential mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 316,785 186,221
Gross Unrealized Gains 0 36
Gross Unrealized Losses (11,367) (2,799)
Available-for-sale securities, debt securities 305,418 183,458
Agency-guaranteed commercial real estate mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost   238,809
Gross Unrealized Gains   432
Gross Unrealized Losses   (769)
Available-for-sale securities, debt securities   238,472
Equity securities    
Debt Securities, Available-for-sale [Line Items]    
Investment securities, at fair value $ 1,819  
Amortized Cost   2,311
Gross Unrealized Gains   1,041
Gross Unrealized Losses   0
Investment securities, at fair value   $ 3,352