Investment securities (Details 4) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Held To Maturity Securities Continuous Unrealized Loss Position [Abstract]
Held to maturity, Estimated fair value, Less than 12 months	$ 5,598	$ 4,967
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses	(23)	(596)
Held to maturity, Estimated fair value, 12 months or more	157,828	168,569
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses		(98,572)
Investment securities in continuous unrealized loss position
Total investment securities, fair value less than 12 months	175,926	425,983
Investment Securities Continuous Unrealized Loss Position Less Than Twelve Months Aggregate Losses	(5,436)	(29,692)
Total of investment securities, fair value, more than 12 months	1,104,080	1,020,418
Investment Securities Continuous Unrealized Loss Position Twelve Months or Longer Aggregate Losses	(318,951)	(363,340)
Obligations of states and political subdivisions [Member]
Held To Maturity Securities Continuous Unrealized Loss Position [Abstract]
Held to maturity, Estimated fair value, Less than 12 months	5,598	3,084
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses	(23)	(4)
Held to maturity, Estimated fair value, 12 months or more	169	1,430
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses	(7)	(24)
Privately Issued Mortgage Backed Securities [Member]
Held To Maturity Securities Continuous Unrealized Loss Position [Abstract]
Held to maturity, Estimated fair value, Less than 12 months		1,883
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses		(592)
Held to maturity, Estimated fair value, 12 months or more	157,659	167,139
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses	$ (97,220)	$ (98,548)