Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) (1) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Investment Securities [Line Items]
Amortized Cost	$ 880,416	$ 896,495
Securities available for sale, at fair value	908,236	902,292
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	27,832	8,906
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	12	3,109
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	100,993	1,000
Securities available for sale, at fair value	100,993	1,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	2	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2	$ 0
US Government Agencies Residential Mortgage-backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	216,000
Securities available for sale, at fair value	226,100
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	527,600
Securities available for sale, at fair value	543,500
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	880,416
Securities available for sale, at fair value	$ 908,236

Investment Securities [Line Items]

No definition available.

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- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

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- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

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.

+ References

No definition available.

+ Details

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