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
	Mar. 31, 2013	Mar. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives not designated as hedging instruments, Derivatives	$ 587	$ 429
Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives not designated as hedging instruments, Derivatives	968	1,139
Foreign exchange and other option and futures contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives not designated as hedging instruments, Derivatives	(381)	(710)
Interest rate swap agreements [Member] | Fixed rate long-term borrowings [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives in fair value hedging relationships, Derivatives	(8,873)	(7,029)
Derivatives in fair value hedging relationships, Hedged item	$ 8,900	$ 6,544