Fair Value Measurements - Estimated Fair Values of Company's Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
FINANCIAL ASSETS
Debt securities held for investment (fair value: $2,953 and $2,984, respectively)	$ 2,956	$ 2,964
Loans, net of allowance for credit losses, Carrying Amount	6,267,530	6,303,140
Debt securities held for investment, Fair Value	2,953	2,984
Loans held for sale, Fair Value	21,718	53,719
FINANCIAL LIABILITIES
Deposits, Carrying Amount	9,371,940	8,064,704
Short-term borrowings, Carrying Amount	3,750	1,100
Junior subordinated debentures, Carrying Amount	26,804	26,804
OFF-BALANCE SHEET FINANCIAL INSTRUMENTS
Loan commitments, Fair Value	3,237	3,115
Letters of credit, Fair Value	542	722
Level 2 Inputs [Member]
FINANCIAL ASSETS
Cash and cash equivalents, Carrying Amount	3,062,408	1,616,912
Debt securities held for investment (fair value: $2,953 and $2,984, respectively)	51	59
Loans held for sale, Carrying Amount	21,718	53,719
Cash and cash equivalents, Fair Value	3,062,408	1,616,912
Debt securities held for investment, Fair Value	53	62
Loans held for sale, Fair Value	21,718	53,719
FINANCIAL LIABILITIES
Deposits, Carrying Amount	9,371,940	8,064,704
Short-term borrowings, Carrying Amount	3,750	1,100
Junior subordinated debentures, Carrying Amount	26,804	26,804
Deposits, Fair Value	9,394,515	8,084,695
Short-term borrowings, Fair Value	3,750	1,100
Junior subordinated debentures, Fair Value	28,948	30,535
Level 3 Inputs [Member]
FINANCIAL ASSETS
Debt securities held for investment (fair value: $2,953 and $2,984, respectively)	2,905	2,905
Loans, net of allowance for credit losses, Carrying Amount	6,267,530	6,303,140
Debt securities held for investment, Fair Value	2,900	2,922
Loans, net of allowance for credit losses, Fair Value	$ 6,269,096	$ 6,347,803