Investment Securities - Additional Information (Detail)	3 Months Ended
Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments	$ 0	$ 0
Gain Losses Maturities And Impairment	6,000	19,000
Available-for-sale Securities, Gross Realized Gains	0	0
Gross Losses	0	0
Gross Gains	6,000	19,000
Debt Securities, Held-to-maturity, Restricted	305,800,000		$ 228,100,000
Proceeds from Sale of Available-for-sale Securities	0	0
Debt Securities, Available-for-sale, Restricted	$ 621,900,000		792,100,000
Investment Securities in Unrealized Loss Position, Qualitative Disclosure, Number of Positions, Total	49
Amortized Cost	$ 1,033,361,000		805,515,000
Available-for-sale Securities	1,043,258,000		831,223,000
Other than temporary impairment charges recognized	0	0
Other Investments and Securities, at Cost	1,700,000		1,700,000
Federal Home Loan Bank Stock	$ 11,800,000		10,600,000
US Government-sponsored Enterprises Debt Securities [Member]
Percent of Unrealized Losses	88.00%
Available-for-sale Securities	$ 975,000		$ 100,998,000
Corporate Debt Securities [Member]
Investment Securities in Unrealized Loss Position, Qualitative Disclosure, Number of Positions, Total	1		0
Amortized Cost	$ 23,100,000		$ 22,900,000
Available-for-sale Securities	24,459,000		$ 24,385,000
Equity Securities [Member]
Other than temporary impairment charges recognized	$ 0	$ 0

Gain losses maturities and impairment.

No definition available.

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This item represents the gross gain on maturities of available for sale securities.

+ References

No definition available.

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This item represents the gross loss on maturities of available for sale securities.

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No definition available.

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Investment Securities in Unrealized Loss Position, Qualitative Disclosure, Number of Positions, Total

+ References

No definition available.

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Percent of Unrealized Losses

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No definition available.

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- Definition

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

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No definition available.

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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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No definition available.

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This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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No definition available.

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This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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No definition available.

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Amount of restricted investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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- Definition

Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Amount of restricted investment in debt security measured at amortized cost (held-to-maturity).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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- Definition

Includes other investments and securities that are not at fair value as of the balance sheet date and those that are not accounted for under the equity method.

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- Definition

The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

+ References

No definition available.

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The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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