Credit Risk Profile by Risk Rating (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	$ 5,389,763	$ 4,960,343	$ 4,776,071
Commercial
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	2,691,240	2,234,817
Commercial | Non-watch list
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	2,471,369	2,064,658
Commercial | Watch
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	104,327	100,499
Commercial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	38,369	16,688
Commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	77,175	52,972
Real estate - construction
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	73,391	84,590
Real estate - construction | Non-watch list
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	70,349	83,100
Real estate - construction | Watch
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	416	355
Real estate - construction | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	2,626	1,135
Real estate - commercial
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	1,342,136	1,394,555
Real estate - commercial | Non-watch list
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	1,233,619	1,275,280
Real estate - commercial | Watch
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	54,106	27,777
Real estate - commercial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	9,378	35,019
Real estate - commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	$ 45,033	$ 56,479