Fair Values of Financial Instruments - Carrying Amount and Fair Value of Company's Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Financial assets:
Securities held-to-maturity	$ 1,565,191	$ 1,438,903
Loans	1,816,885	1,708,461
Financial liabilities:
Time deposits	454,631	473,426
Other borrowed funds	293,000	368,000
Subordinated debentures	36,083	36,083
Carrying Value [Member]
Financial assets:
Securities held-to-maturity	1,565,191	1,438,903
Loans	1,816,885	1,708,461
Financial liabilities:
Time deposits	454,631	473,426
Other borrowed funds	293,000	368,000
Subordinated debentures	36,083	36,083
Estimated Fair Value [Member]
Financial assets:
Securities held-to-maturity	1,589,183	1,438,960
Loans	1,830,287	1,677,270
Financial liabilities:
Time deposits	457,797	474,046
Other borrowed funds	302,163	372,209
Subordinated debentures	36,083	36,083
Fair Value Measurements, Level 2 Inputs [Member]
Financial assets:
Securities held-to-maturity	1,589,183	1,438,960
Financial liabilities:
Time deposits	457,797	474,046
Other borrowed funds	302,163	372,209
Fair Value Measurements, Level 3 Inputs [Member]
Financial assets:
Loans	1,830,287	1,677,270
Financial liabilities:
Subordinated debentures	$ 36,083	$ 36,083