Fair Values of Financial Instruments - Carrying Amount and Fair Value of Company's Financial Instruments (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Financial assets:
Securities held-to-maturity	$ 1,573,829	$ 1,438,903
Loans held-for-sale	12,403
Loans	1,778,041	1,708,461
Financial liabilities:
Time deposits	460,527	473,426
Other borrowed funds	431,000	368,000
Subordinated debentures	36,083	36,083
Carrying Value [Member]
Financial assets:
Securities held-to-maturity	1,573,829	1,438,903
Loans held-for-sale	12,403
Loans	1,778,041	1,708,461
Financial liabilities:
Time deposits	460,527	473,426
Other borrowed funds	431,000	368,000
Subordinated debentures	36,083	36,083
Estimated Fair Value [Member]
Financial assets:
Securities held-to-maturity	1,606,758	1,438,960
Loans held-for-sale	12,704
Loans	1,808,359	1,677,270
Financial liabilities:
Time deposits	464,613	474,046
Other borrowed funds	439,956	372,209
Subordinated debentures	36,083	36,083
Fair Value Measurements, Level 2 Inputs [Member]
Financial assets:
Securities held-to-maturity	1,606,758	1,438,960
Loans held-for-sale	12,704
Financial liabilities:
Time deposits	464,613	474,046
Other borrowed funds	439,956	372,209
Fair Value Measurements, Level 3 Inputs [Member]
Financial assets:
Loans	1,808,359	1,677,270
Financial liabilities:
Subordinated debentures	$ 36,083	$ 36,083