Derivative financial instruments - Information About Fair Values of Derivative Instruments in Consolidated Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	$ 603,019	$ 619,001
Liability Derivatives, Fair Value	384,866	440,685
Derivatives designated and qualifying as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	144,293	147,534
Liability Derivatives, Fair Value	3,825	2,287
Derivatives not designated and qualifying as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	458,726	471,467
Liability Derivatives, Fair Value	381,041	438,398
Interest rate swap agreements [Member] | Derivatives designated and qualifying as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	143,179	147,302
Liability Derivatives, Fair Value
Commitments to sell real estate loans [Member] | Derivatives designated and qualifying as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	1,114	232
Liability Derivatives, Fair Value	3,825	2,287
Commitments to sell real estate loans [Member] | Derivatives not designated and qualifying as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	1,982	1,328
Liability Derivatives, Fair Value	6,570	2,771
Mortgage-related commitments to originate real estate loans for sale [Member] | Derivatives not designated and qualifying as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	48,056	7,991
Liability Derivatives, Fair Value	197	1,068
Interest rate contracts [Member] | Derivatives not designated and qualifying as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	399,963	443,033
Liability Derivatives, Fair Value	365,616	415,836
Foreign exchange and other option and futures contracts [Member] | Derivatives not designated and qualifying as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	8,725	19,115
Liability Derivatives, Fair Value	$ 8,658	$ 18,723