Derivative financial instruments - Information About Fair Values of Derivative Instruments in Consolidated Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Asset derivatives, fair value	$ 681,483	$ 619,001
Liability derivatives, fair value	464,110	440,685
Derivatives designated and qualifying as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives, fair value	153,347	147,534
Liability derivatives, fair value	16,804	2,287
Derivatives designated and qualifying as hedging instruments [Member] | Interest rate swap agreements [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives, fair value	153,220	147,302
Liability derivatives, fair value
Derivatives designated and qualifying as hedging instruments [Member] | Commitments to sell real estate loans [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives, fair value	127	232
Liability derivatives, fair value	16,804	2,287
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives, fair value	528,136	471,467
Liability derivatives, fair value	447,306	438,398
Not Designated as Hedging Instrument [Member] | Commitments to sell real estate loans [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives, fair value	2,777	1,328
Liability derivatives, fair value	22,909	2,771
Not Designated as Hedging Instrument [Member] | Mortgage-related commitments to originate real estate loans for sale [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives, fair value	70,430	7,991
Liability derivatives, fair value	85	1,068
Not Designated as Hedging Instrument [Member] | Interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives, fair value	441,467	443,033
Liability derivatives, fair value	410,929	415,836
Not Designated as Hedging Instrument [Member] | Foreign exchange and other option and futures contracts [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives, fair value	13,462	19,115
Liability derivatives, fair value	$ 13,383	$ 18,723