Derivative financial instruments (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Information about fair values of derivative instruments in consolidated statement of income
Derivatives not designated as hedging instruments, derivatives	$ (355)	$ 258	$ 74	$ 185
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	9,674	21,945	2,645	9,540
Derivatives in fair value hedging relationships, hedged item	(9,168)	(21,145)	(2,624)	(9,097)
Interest rate contracts [Member]
Information about fair values of derivative instruments in consolidated statement of income
Derivatives not designated as hedging instruments, derivatives	2,080	1,001	3,219	1,476
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	$ (2,435)	$ (743)	$ (3,145)	$ (1,291)