Estimated Fair Value of Financial Instruments (Detail) (USD $)	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity	$ 98,479,000	$ 89,246,000
Federal Reserve Bank and other stock	25,526,000	22,212,000
Time deposits	1,126,400,000	1,548,400,000
Long-term debt	5,632,000	6,529,000
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity	113,221,000	102,300,000
Federal Reserve Bank and other stock	25,526,000	22,200,000
Loans (exclusive of allowance for loan loss)	5,477,200,000	5,042,000,000
Time deposits	1,133,800,000	1,557,800,000
Long-term debt	5,800,000	6,800,000
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans (exclusive of allowance for loan loss)	5,403,700,000	4,898,500,000
Total Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity	113,221,000	102,300,000
Federal Reserve Bank and other stock	25,526,000	22,200,000
Loans (exclusive of allowance for loan loss)	5,477,200,000	5,042,000,000
Time deposits	1,133,800,000	1,557,800,000
Long-term debt	5,800,000	6,800,000
Commitments to extend credit for loans	4,200,000	5,800,000
Commercial letters of credit	200,000	300,000
Standby letters of credit	$ 1,500,000	$ 2,200,000