Fair Value Measurements - Estimated Fair Values of Company's Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
FINANCIAL ASSETS
Debt securities held for investment (fair value: $2,984 and $2,984, respectively)	$ 2,982	$ 2,964
Loans, net of allowance for credit losses, Carrying Amount	5,930,473	6,303,140
Debt securities held for investment, Fair Value	2,984	2,984
Loans held for sale, Fair Value	20,950	53,719
FINANCIAL LIABILITIES
Deposits, Carrying Amount	9,992,044	8,064,704
Short-term borrowings, Carrying Amount	3,500	1,100
Subordinated debt, Carrying Amount	85,973	26,804
OFF-BALANCE SHEET FINANCIAL INSTRUMENTS
Loan commitments, Fair Value	3,584	3,115
Letters of credit, Fair Value	560	722
Level 2 Inputs [Member]
FINANCIAL ASSETS
Cash and cash equivalents, Carrying Amount	4,110,866	1,616,912
Debt securities held for investment (fair value: $2,984 and $2,984, respectively)	37	59
Loans held for sale, Carrying Amount	20,950	53,719
Cash and cash equivalents, Fair Value	4,110,866	1,616,912
Debt securities held for investment, Fair Value	39	62
Loans held for sale, Fair Value	20,950	53,719
FINANCIAL LIABILITIES
Deposits, Carrying Amount	9,992,044	8,064,704
Short-term borrowings, Carrying Amount	3,500	1,100
Subordinated debt, Carrying Amount	85,973	26,804
Deposits, Fair Value	10,064,348	8,084,695
Short-term borrowings, Fair Value	3,500	1,100
Subordinated debt, Fair Value	88,072	30,535
Level 3 Inputs [Member]
FINANCIAL ASSETS
Debt securities held for investment (fair value: $2,984 and $2,984, respectively)	2,945	2,905
Loans, net of allowance for credit losses, Carrying Amount	5,930,473	6,303,140
Debt securities held for investment, Fair Value	2,945	2,922
Loans, net of allowance for credit losses, Fair Value	$ 5,919,882	$ 6,347,803