Fair Value Measurement and Fair Value of Financial Instruments- Carrying Values and Fair Values of Company's Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Financial Assets:
Amortized Cost	$ 693,562	$ 90,766
Investment securities held to maturity, fair value	686,728	93,868
Federal Home Loan Bank and other membership bank stocks, Carrying Value	9,340	11,979
Federal Home Loan Bank and other membership bank stocks, Fair Value	9,340	11,979
Loans	5,822,849	5,950,108
Fair Value	5,844,767	5,939,413
Financial Liabilities:
Certificates of deposit, Carrying Value	804,899	1,078,256
Certificates of deposit, Fair Value	800,083	1,077,620
Other borrowings, Carrying Value	25,000	25,000
Other borrowings, Fair Value	24,902	25,206
Subordinated debt, Carrying Value	187,107	118,257
Subordinated debentures, Fair Value	185,965	118,208
U.S. Treasury and U.S. government agencies
Financial Assets:
Amortized Cost	18,820	25,565
Investment securities held to maturity, fair value	19,279	26,344
Mortgage-backed securities, residential
Financial Assets:
Amortized Cost	343,866	39,276
Investment securities held to maturity, fair value	342,001	40,733
Collateralized mortgage obligations, residential
Financial Assets:
Amortized Cost	8,488	14,590
Investment securities held to maturity, fair value	8,775	15,122
Mortgage-backed securities, multifamily
Financial Assets:
Amortized Cost	2,724	705
Investment securities held to maturity, fair value	2,771	759
Obligations of states and political subdivisions
Financial Assets:
Amortized Cost	316,832	10,630
Investment securities held to maturity, fair value	311,037	10,910
Corporate bonds
Financial Assets:
Amortized Cost	2,832
Investment securities held to maturity, fair value	2,865
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial Assets:
Investment securities held to maturity, fair value	0	0
Federal Home Loan Bank and other membership bank stocks, Fair Value	0	0
Fair Value	0	0
Financial Liabilities:
Certificates of deposit, Fair Value	0	0
Other borrowings, Fair Value	0	0
Subordinated debentures, Fair Value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| U.S. Treasury and U.S. government agencies
Financial Assets:
Investment securities held to maturity, fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Mortgage-backed securities, residential
Financial Assets:
Investment securities held to maturity, fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Collateralized mortgage obligations, residential
Financial Assets:
Investment securities held to maturity, fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Mortgage-backed securities, multifamily
Financial Assets:
Investment securities held to maturity, fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Obligations of states and political subdivisions
Financial Assets:
Investment securities held to maturity, fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Corporate bonds
Financial Assets:
Investment securities held to maturity, fair value	0
Significant Other Observable Inputs (Level 2)
Financial Assets:
Investment securities held to maturity, fair value	686,269	93,868
Federal Home Loan Bank and other membership bank stocks, Fair Value	9,340	11,979
Fair Value	0	0
Financial Liabilities:
Certificates of deposit, Fair Value	800,083	1,077,620
Other borrowings, Fair Value	24,902	25,206
Subordinated debentures, Fair Value	0	0
Significant Other Observable Inputs (Level 2) \| U.S. Treasury and U.S. government agencies
Financial Assets:
Investment securities held to maturity, fair value	19,279	26,344
Significant Other Observable Inputs (Level 2) \| Mortgage-backed securities, residential
Financial Assets:
Investment securities held to maturity, fair value	342,001	40,733
Significant Other Observable Inputs (Level 2) \| Collateralized mortgage obligations, residential
Financial Assets:
Investment securities held to maturity, fair value	8,775	15,122
Significant Other Observable Inputs (Level 2) \| Mortgage-backed securities, multifamily
Financial Assets:
Investment securities held to maturity, fair value	2,771	759
Significant Other Observable Inputs (Level 2) \| Obligations of states and political subdivisions
Financial Assets:
Investment securities held to maturity, fair value	310,578	10,910
Significant Other Observable Inputs (Level 2) \| Corporate bonds
Financial Assets:
Investment securities held to maturity, fair value	2,865
Significant Unobservable Inputs (Level 3)
Financial Assets:
Investment securities held to maturity, fair value	459	0
Federal Home Loan Bank and other membership bank stocks, Fair Value	0	0
Fair Value	5,844,767	5,939,413
Financial Liabilities:
Certificates of deposit, Fair Value	0	0
Other borrowings, Fair Value	0	0
Subordinated debentures, Fair Value	185,965	118,208
Significant Unobservable Inputs (Level 3) \| U.S. Treasury and U.S. government agencies
Financial Assets:
Investment securities held to maturity, fair value	0	0
Significant Unobservable Inputs (Level 3) \| Mortgage-backed securities, residential
Financial Assets:
Investment securities held to maturity, fair value	0	0
Significant Unobservable Inputs (Level 3) \| Collateralized mortgage obligations, residential
Financial Assets:
Investment securities held to maturity, fair value	0	0
Significant Unobservable Inputs (Level 3) \| Mortgage-backed securities, multifamily
Financial Assets:
Investment securities held to maturity, fair value	0	0
Significant Unobservable Inputs (Level 3) \| Obligations of states and political subdivisions
Financial Assets:
Investment securities held to maturity, fair value	459	$ 0
Significant Unobservable Inputs (Level 3) \| Corporate bonds
Financial Assets:
Investment securities held to maturity, fair value	$ 0

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Other Borrowings Fair Value Disclosure

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Amount excluding accrued interest, after allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of subordinated debt obligations including, but not limited to, subordinated loan, subordinated bond, subordinated debenture or junior debt.

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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).

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Amount excluding accrued interest, after allowance for credit loss, of financing receivable.

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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Fair value portion of an agreement for an unconditional promise by the maker to pay the holder a definite sum of money at a future date.

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