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Investment Securities - Summary of Amortized Cost and Approximate Fair Value of Investment Securities (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 1,214,367 $ 472,300
Gross Unrealized Gains 947 2,639
Gross Unrealized Losses (37,015) (3,568)
Available-for-sale securities, debt securities 1,178,299  
Investment securities, at fair value 1,181,661 471,371
Fair value 1,181,661 471,371
Corporate notes    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 374,611 44,959
Gross Unrealized Gains 947 1,130
Gross Unrealized Losses (15,050) 0
Available-for-sale securities, debt securities 360,508  
Fair value   46,089
Agency-guaranteed residential mortgage-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 505,113 186,221
Gross Unrealized Gains 0 36
Gross Unrealized Losses (11,856) (2,799)
Available-for-sale securities, debt securities 493,257  
Fair value   183,458
Agency-guaranteed commercial real estate mortgage-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 334,643 238,809
Gross Unrealized Gains 0 432
Gross Unrealized Losses (10,109) (769)
Available-for-sale securities, debt securities 324,534  
Fair value   238,472
Equity securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost   2,311
Gross Unrealized Gains   1,041
Gross Unrealized Losses   0
Investment securities, at fair value $ 3,362  
Fair value   $ 3,352