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Investment Securities - Summary of Amortized Cost and Approximate Fair Value of Investment Securities (Detail) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]      
Amortized Cost $ 687,985 $ 692,674  
Gross Unrealized Gains 701 920  
Gross Unrealized Losses (12,264) (30,300)  
Fair Value 676,422 663,294  
Equity securities 1,720   $ 1,718
Investment securities, at fair value 678,142 665,012 $ 665,012
Agency-guaranteed residential mortgage-backed securities      
Debt Securities, Available-for-sale [Line Items]      
Amortized Cost 306,651 311,267  
Gross Unrealized Gains 62 0  
Gross Unrealized Losses (2,569) (5,893)  
Fair Value 304,144 305,374  
Corporate notes      
Debt Securities, Available-for-sale [Line Items]      
Amortized Cost 381,334 381,407  
Gross Unrealized Gains 639 920  
Gross Unrealized Losses (9,695) (24,407)  
Fair Value $ 372,278 $ 357,920