Investment Securities - Maturity of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Net Investment Income [Line Items]
Due within one year, Amortized Cost	$ 5,000
Due within one year, Estimated Fair Value	5,027
Due after one but within five years, Amortized Cost	13,872
Due after one but within five years, Estimated Fair Value	14,498
Due after five but within ten years, Amortized Cost	19,821
Due after five but within ten years, Estimated Fair Value	20,316
Due after ten years, Amortized Cost	0
Due after ten years, Estimated Fair Value	0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	38,693
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value	39,841
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	1,078,691
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	1,085,115
Amortized Cost	1,117,384	$ 805,515
Securities available for sale, at fair value	1,124,956	$ 831,223
Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost	1,117,384
Securities available for sale, at fair value	1,124,956
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost	396,000
Securities available for sale, at fair value	399,700
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Amortized Cost	682,700
Securities available for sale, at fair value	$ 685,400

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), without single maturity date and not allocated over maturity grouping.

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), without single maturity date and not allocated over maturity grouping.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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