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Investment Securities - Additional Information (Detail) - USD ($)
Sep. 30, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Investment securities pledged as collateral $ 1,600,000,000 $ 1,910,000,000
Investment in FHLB stock, impairment losses $ 0  
United States [Member]    
Debt Securities, Available-for-sale [Line Items]    
Securities issued by U.S. government percentage 89.00%