Derivative financial instruments - Information About Fair Values of Derivative Instruments in Consolidated Statement of Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives not designated as hedging instruments, derivatives	$ 851	$ 4,189	$ 925	$ 4,374
Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives not designated as hedging instruments, derivatives	777	2,425	3,996	3,901
Foreign exchange and other option and futures contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives not designated as hedging instruments, derivatives	74	1,764	(3,071)	473
Interest rate swap agreements [Member] | Fixed rate long-term borrowings [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives in fair value hedging relationships, derivatives	3,273	42,587	5,918	52,127
Derivatives in fair value hedging relationships, hedged item	$ (3,252)	$ (40,355)	$ (5,876)	$ (49,452)