Derivative financial instruments (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Information about fair values of derivative instruments in consolidated statement of income
Derivatives not designated as hedging instruments, derivatives	$ 429	$ (73)
Interest rate swap agreements [Member] | Fixed rate long-term borrowings [Member]
Information about fair values of derivative instruments in consolidated statement of income
Derivatives in fair value hedging relationships, derivatives	(7,029)	(12,405)
Derivatives in fair value hedging relationships, hedged item	6,544	12,048
Interest rate contracts [Member]
Information about fair values of derivative instruments in consolidated statement of income
Derivatives not designated as hedging instruments, derivatives	1,139	475
Foreign exchange and other option and futures contracts [Member]
Information about fair values of derivative instruments in consolidated statement of income
Derivatives not designated as hedging instruments, derivatives	$ (710)	$ (548)