Credit Risk Portfolio by Risk Rating (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	$ 6,010,681	$ 5,686,749	$ 5,144,766
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	3,186,562	2,873,694
Commercial [Member] | Non-watch list [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	3,008,416	2,670,925
Commercial [Member] | Watch [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	84,534	98,636
Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	32,683	29,462
Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	60,929	74,671
Real estate - construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	84,868	78,486
Real estate - construction [Member] | Non-watch list [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	81,274	75,631
Real estate - construction [Member] | Watch [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	873	518
Real estate - construction [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	14	14
Real estate - construction [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	2,707	2,323
Real estate - commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	1,443,263	1,435,811
Real estate - commercial [Member] | Non-watch list [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	1,328,480	1,325,460
Real estate - commercial [Member] | Watch [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	65,234	63,278
Real estate - commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	16,892	11,613
Real estate - commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit Risk Profile by Risk Rating	$ 32,657	$ 35,460