Estimated Fair Value of Financial Instruments (Detail) (USD $)	Jun. 30, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity	$ 238,799,000	$ 209,770,000
Federal Reserve Bank and other stock	67,527,000	50,482,000
Loans (exclusive of allowance for loan loss)	6,923,800,000	6,521,900,000
Time deposits	1,020,000,000	1,449,600,000
Long-term debt	5,745,000	5,055,000
Total Estimated Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity	269,700,000	231,500,000
Federal Reserve Bank and other stock	67,500,000	50,500,000
Loans (exclusive of allowance for loan loss)	6,937,500,000	6,571,600,000
Time deposits	1,019,500,000	1,449,400,000
Long-term debt	5,900,000	4,500,000
Commitments to extend credit for loans	2,900,000	6,000,000
Commercial letters of credit	100,000	100,000
Standby letters of credit	1,200,000	2,000,000
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity	269,700,000	231,500,000
Federal Reserve Bank and other stock	67,500,000	50,500,000
Loans (exclusive of allowance for loan loss)	6,937,500,000	6,571,600,000
Time deposits	1,019,500,000	1,449,400,000
Long-term debt	$ 5,900,000	$ 4,500,000