Estimated Fair Value of Financial Instruments (Detail) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Held to maturity	$ 129,498,000	$ 114,756,000
Federal Reserve Bank stock and other	26,127,000	26,333,000
Loans (exclusive of allowance for loan loss)	6,017,000,000	5,690,600,000
Time deposits	1,066,600,000	1,282,300,000
Long-term debt	4,393,000	5,879,000
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Held to maturity	129,500,000
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Held to maturity	129,500,000	129,498,000
Federal Reserve Bank stock and other	26,100,000	26,300,000
Loans (exclusive of allowance for loan loss)	6,086,900,000	5,754,100,000
Time deposits	1,071,700,000	1,287,900,000
Long-term debt	4,600,000	6,100,000
Total Estimated Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Held to maturity	129,500,000	129,498,000
Federal Reserve Bank stock and other	26,100,000	26,300,000
Loans (exclusive of allowance for loan loss)	6,086,900,000	5,754,100,000
Time deposits	1,071,700,000	1,287,900,000
Long-term debt	4,600,000	6,100,000
Commitments to extend credit for loans	1,400,000	5,600,000
Commercial letters of credit	100,000	200,000
Standby letters of credit	400,000	2,100,000
Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans (exclusive of allowance for loan loss)	$ 6,017,000,000	$ 5,690,600,000