Fair Value Measurements - Estimated Fair Values of Company's Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
FINANCIAL ASSETS
Securities held for investment, Carrying Amount	$ 1,440	$ 2,292
Loans, net of allowance for loan losses, Carrying Amount	4,895,653	4,670,329
Securities held for investment, Fair Value	1,447	2,303
FINANCIAL LIABILITIES
Deposits, Carrying Amount	6,643,131	6,415,045
Short-term borrowings, Carrying Amount	2,200	900
Junior subordinated debentures, Carrying Amount	31,959	31,959
OFF-BALANCE SHEET FINANCIAL INSTRUMENTS
Loan commitments, Fair Value	2,006	1,839
Letters of credit, Fair Value	425	428
Level 2 Inputs [Member]
FINANCIAL ASSETS
Cash and cash equivalents, Carrying Amount	1,792,766	1,758,575
Securities held for investment, Carrying Amount	940	1,792
Cash and cash equivalents, Fair Value	1,792,766	1,758,575
Securities held for investment, Fair Value	947	1,803
FINANCIAL LIABILITIES
Deposits, Carrying Amount	6,643,131	6,415,045
Short-term borrowings, Carrying Amount	2,200	900
Junior subordinated debentures, Carrying Amount	31,959	31,959
Deposits, Fair Value	6,760,191	6,490,309
Short-term borrowings, Fair Value	2,200	900
Junior subordinated debentures, Fair Value	34,795	34,661
Level 3 Inputs [Member]
FINANCIAL ASSETS
Securities held for investment, Carrying Amount	500	500
Loans, net of allowance for loan losses, Carrying Amount	4,895,653	4,670,329
Securities held for investment, Fair Value	500	500
Loans, net of allowance for loan losses, Fair Value	$ 4,854,167	$ 4,663,608