Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) (1) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment Securities [Line Items]
Amortized Cost	$ 898,877	$ 924,054
Securities available for sale, at fair value	893,101	909,247
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	4,620	2,998
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	10,396	17,805
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	10,000	10,000
Securities available for sale, at fair value	10,006	10,012
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	6	12
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	$ 0
US Government Agencies Residential Mortgage-backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	177,900
Securities available for sale, at fair value	177,400
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	660,100
Securities available for sale, at fair value	654,000
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	898,877
Securities available for sale, at fair value	$ 893,101

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=118939692&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=118939692&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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- Details

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