Estimated Fair Value of Financial Instruments (Detail) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity	$ 114,756,000	$ 89,246,000
Federal Reserve Bank and other stock	26,333,000	22,212,000
Loans (exclusive of allowance for loan loss)	5,690,600,000	4,898,500,000
Time deposits	1,282,300,000	1,548,400,000
Long-term debt	5,879,000	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	129,500,000	102,300,000
Federal Reserve Bank and other stock	26,300,000	22,200,000
Loans (exclusive of allowance for loan loss)	5,754,100,000	5,042,000,000
Time deposits		1,557,800,000
Long-term debt		6,800,000
Total Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity	129,500,000	102,300,000
Federal Reserve Bank and other stock	26,300,000	22,200,000
Loans (exclusive of allowance for loan loss)	5,754,100,000	5,042,000,000
Time deposits	1,287,900,000	1,557,800,000
Long-term debt	6,100,000	6,800,000
Commitments to extend credit for loans	5,600,000	5,800,000
Commercial letters of credit	200,000	300,000
Standby letters of credit	2,100,000	2,200,000
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans (exclusive of allowance for loan loss)	$ 5,690,600,000	$ 4,898,500,000