Derivative financial instruments (Details 2) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Information about fair values of derivative instruments in consolidated statement of income
Derivatives not designated as hedging instruments, derivatives	$ 4,189	$ (719)	$ 4,374	$ (880)
Derivatives not designated as hedging instruments, hedged item	0		0
Interest rate swap agreements [Member]
Information about fair values of derivative instruments in consolidated statement of income
Derivatives in fair value hedging relationships, derivatives	42,587	28,281	52,127	84,205
Derivatives in fair value hedging relationships, hedged item	(40,355)	(27,166)	(49,452)	(80,324)
Interest rate swap agreements [Member] | Fixed rate time deposits [Member]
Information about fair values of derivative instruments in consolidated statement of income
Derivatives in fair value hedging relationships, derivatives	0		0	(503)
Derivatives in fair value hedging relationships, hedged item	0		0	503
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	42,587	28,281	52,127	84,708
Derivatives in fair value hedging relationships, hedged item	(40,355)	(27,166)	(49,452)	(80,827)
Interest rate contracts [Member]
Information about fair values of derivative instruments in consolidated statement of income
Derivatives not designated as hedging instruments, derivatives	2,425	813	3,901	(305)
Derivatives not designated as hedging instruments, hedged item	0		0
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	1,764	(1,532)	473	(575)
Derivatives not designated as hedging instruments, hedged item	$ 0		$ 0