Investment Securities - Maturity of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Net Investment Income [Line Items]
Due within one year, Amortized Cost	$ 5,000
Due within one year, Estimated Fair Value	5,086
Due after one but within five years, Amortized Cost	12,870
Due after one but within five years, Estimated Fair Value	13,670
Due after five but within ten years, Amortized Cost	15,923
Due after five but within ten years, Estimated Fair Value	16,243
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	33,793
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value	34,999
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	999,568
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	1,008,259
Amortized Cost	1,033,361	$ 805,515
Securities available for sale, at fair value	1,043,258	$ 831,223
Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost	1,033,361
Securities available for sale, at fair value	1,043,258
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost	275,000
Securities available for sale, at fair value	279,600
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Amortized Cost	724,600
Securities available for sale, at fair value	$ 728,700

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

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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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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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 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), with single maturity date and allocated without single maturity date, maturing in next 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 next 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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