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Securities - Securities Available for Sale, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	$ 1,000,258
Available for Sale, Less than 12 Months, Unrealized Losses	31,598
Available for Sale, More than 12 Months, Estimated Fair Value	0
Available for Sale, More than 12 Months, Unrealized Losses	0
Available for Sale, Total Estimated Fair Value	1,000,258
Available-for-sale Securities, Gross Unrealized Loss, before Tax	31,598	$ 4
U.S. Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	188,812
Available for Sale, Less than 12 Months, Unrealized Losses	8,202
Available for Sale, More than 12 Months, Estimated Fair Value	0
Available for Sale, More than 12 Months, Unrealized Losses	0
Available for Sale, Total Estimated Fair Value	188,812
Available-for-sale Securities, Gross Unrealized Loss, before Tax	8,202	0
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	634,464
Available for Sale, Less than 12 Months, Unrealized Losses	17,236
Available for Sale, More than 12 Months, Estimated Fair Value	0
Available for Sale, More than 12 Months, Unrealized Losses	0
Available for Sale, Total Estimated Fair Value	634,464
Available-for-sale Securities, Gross Unrealized Loss, before Tax	17,236	4
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	176,982
Available for Sale, Less than 12 Months, Unrealized Losses	6,160
Available for Sale, More than 12 Months, Estimated Fair Value	0
Available for Sale, More than 12 Months, Unrealized Losses	0
Available for Sale, Total Estimated Fair Value	176,982
Available-for-sale Securities, Gross Unrealized Loss, before Tax	$ 6,160	$ 0

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- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

No definition available.

+ Details

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- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

+ References

No definition available.

+ Details

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- Definition

This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

+ References

No definition available.

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- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

+ References

No definition available.

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- Definition

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+ References

No definition available.

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- Definition

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for twelve months or longer.

+ References

No definition available.

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