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Summary of Significant Accounting Policies - Summary of Cash and Cash Equivalents (Detail) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Sep. 30, 2020
Accounting Policies [Abstract]      
Due from the FRB $ 5,701,276   $ 1,578,579
Cash and due from banks 426,843 $ 430,638 440,659
Cash and cash equivalents at end of period $ 6,128,119   $ 2,019,238