Fair Value Measurements - Estimated Fair Values of Company's Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
FINANCIAL ASSETS
Securities held for investment, Carrying Amount	$ 2,292	$ 4,365
Loans, net of allowance for loan losses, Carrying Amount	4,670,329	4,351,539
Securities held for investment, Fair Value	2,303	4,403
FINANCIAL LIABILITIES
Deposits, Carrying Amount	6,415,045	6,248,057
Short-term borrowings, Carrying Amount	900	500
Junior subordinated debentures, Carrying Amount	31,959	31,959
OFF-BALANCE SHEET FINANCIAL INSTRUMENTS
Loan commitments, Fair Value	1,839	1,718
Letters of credit, Fair Value	428	424
Level 2 Inputs [Member]
FINANCIAL ASSETS
Cash and cash equivalents, Carrying Amount	1,758,575	1,851,161
Securities held for investment, Carrying Amount	1,792	3,722
Cash and cash equivalents, Fair Value	1,758,575	1,851,161
Securities held for investment, Fair Value	1,803	3,760
FINANCIAL LIABILITIES
Deposits, Carrying Amount	6,415,045	6,248,057
Short-term borrowings, Carrying Amount	900	500
Junior subordinated debentures, Carrying Amount	31,959	31,959
Deposits, Fair Value	6,490,309	6,313,622
Short-term borrowings, Fair Value	900	500
Junior subordinated debentures, Fair Value	34,661	34,339
Level 3 Inputs [Member]
FINANCIAL ASSETS
Securities held for investment, Carrying Amount	500	643
Loans, net of allowance for loan losses, Carrying Amount	4,670,329	4,351,539
Securities held for investment, Fair Value	500	643
Loans, net of allowance for loan losses, Fair Value	$ 4,663,608	$ 4,367,363