Derivative financial instruments - Information About Fair Values of Derivative Instruments in Consolidated Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Asset derivatives, Fair value	$ 537,507	$ 603,019
Liability derivatives, Fair value	337,142	384,866
Derivatives designated and qualifying as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives, Fair value	137,561	144,293
Liability derivatives, Fair value	3,913	3,825
Derivatives not designated and qualifying as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives, Fair value	399,946	458,726
Liability derivatives, Fair value	333,229	381,041
Interest rate swap agreements [Member] | Derivatives designated and qualifying as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives, Fair value	134,306	143,179
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	3,255	1,114
Liability derivatives, Fair value	3,913	3,825
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	1,664	1,982
Liability derivatives, Fair value	5,574	6,570
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	36,166	48,056
Liability derivatives, Fair value	47	197
Interest rate contracts [Member] | Derivatives not designated and qualifying as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives, Fair value	351,611	399,963
Liability derivatives, Fair value	317,701	365,616
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	10,505	8,725
Liability derivatives, Fair value	$ 9,907	$ 8,658