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Derivative Instruments and Hedging Activities - Cumulative Basis Adjustment for Fair Value Hedges (Detail) - Available-for-sale Securities - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Amortized Cost $ 85,500 $ 272,159
Cumulative Amount of Fair Value Hedging Adjustment to Hedged Items $ 976 $ 741