Derivative financial instruments - Information About Fair Values of Derivative Instruments in consolidated Statement of Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives not designated as hedging instruments, Derivatives	$ 4,034	$ 3,481	$ 3,453
Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives not designated as hedging instruments, Derivatives	8,004	6,130	3,760
Foreign exchange and other option and futures contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives not designated as hedging instruments, Derivatives	(3,970)	(2,649)	(307)
Interest rate swap agreements [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives in fair value hedging relationships, Derivatives	(4,123)	50,665	41,125
Derivatives in fair value hedging relationships, Hedged Item	3,724	(47,916)	(39,299)
Interest rate swap agreements [Member] | Fixed rate time deposits( [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives in fair value hedging relationships, Derivatives			(503)
Derivatives in fair value hedging relationships, Hedged Item			503
Interest rate swap agreements [Member] | Fixed rate long-term borrowings [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives in fair value hedging relationships, Derivatives	(4,123)	50,665	41,628
Derivatives in fair value hedging relationships, Hedged Item	$ 3,724	$ (47,916)	$ (39,802)