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Investment Securities - Summary of Amortized Cost and Approximate Fair Value of Investment Securities (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 1,206,202 $ 472,300
Gross Unrealized Gains 798 2,639
Gross Unrealized Losses (49,056) (3,568)
Available-for-sale securities, debt securities 1,157,944  
Investment securities, at fair value 1,161,000 471,371
Fair value 1,161,000 471,371
Corporate notes    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 381,545 44,959
Gross Unrealized Gains 798 1,130
Gross Unrealized Losses (21,335) 0
Available-for-sale securities, debt securities 361,008  
Fair value   46,089
Agency-guaranteed residential mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 490,425 186,221
Gross Unrealized Gains 0 36
Gross Unrealized Losses (13,862) (2,799)
Available-for-sale securities, debt securities 476,563  
Fair value   183,458
Agency-guaranteed commercial real estate mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 334,232 238,809
Gross Unrealized Gains 0 432
Gross Unrealized Losses (13,859) (769)
Available-for-sale securities, debt securities 320,373  
Fair value   238,472
Equity securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost   2,311
Gross Unrealized Gains   1,041
Gross Unrealized Losses   0
Investment securities, at fair value $ 3,056  
Fair value   $ 3,352