Securities - Additional Information (Detail) $ in Thousands	9 Months Ended
Sep. 30, 2019 USD ($) Holding	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) Holding
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Maximum percentage of equity of one issuer	10.00%
Number of holdings greater than specified percentage of equity \| Holding	0		0
Equities securities	$ 11,644		$ 11,737
Securities with aggregate fair values	2,000,000		2,000,000
Proceeds from sale of available-for-sale securities	125,239	$ 82,134
Net unrealized gains (losses) on available-for-sale securities included in AOCI	24,700		(21,500)
Federal home loan bank stock, Total	56,300		$ 50,800
Grantor Trusts [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Equities securities	$ 8,500

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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=118930883&loc=d3e681-108580

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

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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).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 325
-Section 45
-Paragraph 1
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The carrying amount, as of the date of the latest financial statement presented, of other assets which are owned but transferred to serve as collateral for the payment of the related debt obligation, primarily a secured borrowing or repurchase agreement, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
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The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 11
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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The maximum percentage of shareholders' equity of any one issuer other than the U.S. government and its agencies.

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Number of holdings greater than specified percentage of equity.

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Schedule of available for sale securities and held to maturity.

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