Fair Value Measurements - Estimated Fair Values of Company's Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
FINANCIAL ASSETS
Cash and cash equivalents, Carrying Amount	$ 1,706,156	$ 1,598,177	$ 1,655,061	$ 1,913,895
Securities held for investment, Carrying Amount	4,554	8,789
Loans, net of allowance for loan losses, Carrying Amount	4,259,766	4,190,382
Securities held for investment, Fair Value	4,601	8,850
FINANCIAL LIABILITIES
Deposits, Carrying Amount	6,024,949	5,973,358
Short-term borrowings, Carrying Amount	4,000	500
Junior subordinated debentures, Carrying Amount	31,959	31,959
OFF-BALANCE SHEET FINANCIAL INSTRUMENTS
Loan commitments, Fair value	1,824	1,681
Letters of credit, Fair value	428	464
Level 2 Inputs [Member]
FINANCIAL ASSETS
Cash and cash equivalents, Carrying Amount	1,706,656	1,598,177
Securities held for investment, Carrying Amount	3,943	8,289
Cash and cash equivalents, Fair Value	1,706,656	1,598,177
Securities held for investment, Fair Value	3,990	8,350
FINANCIAL LIABILITIES
Deposits, Carrying Amount	6,024,949	5,973,538
Short-term borrowings, Carrying Amount	4,000	500
Junior subordinated debentures, Carrying Amount	31,959	31,959
Deposits, Fair Value	6,085,613	6,028,012
Short-term borrowings, Fair Value	4,000	500
Junior subordinated debentures, Fair Value	34,757	33,793
Level 3 Inputs [Member]
FINANCIAL ASSETS
Securities held for investment, Carrying Amount	611	500
Loans, net of allowance for loan losses, Carrying Amount	4,259,766	4,190,382
Securities held for investment, Fair Value	611	500
Loans, net of allowance for loan losses, Fair Value	$ 4,325,916	$ 4,222,153