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Investment Securities Available for Sale - Additional Information (Detail)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2020
USD ($)
Security
Mar. 31, 2019
USD ($)
Dec. 31, 2019
USD ($)
Security
Investments Debt And Equity Securities [Abstract]      
Proceeds from available for sale debt securities sold $ 0 $ 66,486  
Tax provisions related to net realized gains 0 $ 4  
Securities estimated fair value $ 1,403,169   $ 1,195,664
Debt Securities, Held-to-maturity, Restricted [Extensible List] us-gaap:CollateralPledgedMember   us-gaap:CollateralPledgedMember
Securities to third party dealers $ 605,100   $ 361,127
Percentage of AFS securities held by any one issuer as a percentage of stockholders' equity 10.00%   10.00%
Number of securities representing specified criteria | Security 0   0
Percentage of mortgage-backed securities held from U.S. government-sponsored entities and agencies 100.00%