Fair Value Disclosures of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale	$ 1,944,914,000	$ 1,205,079,000
Unrealized gains on equity investments	$ 26,175,000	20,745,000	$ 0
Minimum discount rate on appraisals of collateral securing loan (as percent)	10.00%
Maximum discount rate on appraisals of collateral securing loan (as percent)	20.00%
Gross amount of impaired loans	$ 44,500,000	49,400,000
Related Reserve	2,242,000	2,258,000
Commitments of lending operations	0	$ 0
Accounting Standards Update 2016-01
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unrealized gains on equity investments	26,200,000
Commitments to Extend Credit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commitments of lending operations	2,100,000,000
Significant Other Observable Inputs (Level 2) \| Federal Agency MBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale	$ 1,900,000,000

Amount of 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/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

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Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity.

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

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-Name Accounting Standards Codification
-Topic 321
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Financing Receivable Individually Evaluated For Impairment And Acquired Impaired Loans Fair Value Disclosure

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Impaired Loans Related Specific Reserve

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