Investment securities (Details 3) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Investment Securities Continuous Unrealized Loss Position Fair Value
Estimated fair value, Less than 12 months	$ 170,328	$ 421,016
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(5,413)	(29,096)
Estimated fair value, 12 months or more	946,252	851,849
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(221,724)	(264,768)
U.S. Treasury and federal agencies [Member]
Investment Securities Continuous Unrealized Loss Position Fair Value
Estimated fair value, Less than 12 months	171
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(1)
Estimated fair value, 12 months or more	678
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(7)
Obligations of states and political subdivisions [Member]
Investment Securities Continuous Unrealized Loss Position Fair Value
Estimated fair value, Less than 12 months
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses
Estimated fair value, 12 months or more		1,228
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses		(20)
Government issued or guaranteed [Member]
Investment Securities Continuous Unrealized Loss Position Fair Value
Estimated fair value, Less than 12 months	15,936	38,492
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(70)	(190)
Estimated fair value, 12 months or more	9,951	6,017
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(157)	(85)
Privately issued [Member]
Investment Securities Continuous Unrealized Loss Position Fair Value
Estimated fair value, Less than 12 months		297,133
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses		(14,188)
Estimated fair value, 12 months or more		751,077
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses		(225,300)
Privately issued residential [Member]
Investment Securities Continuous Unrealized Loss Position Fair Value
Estimated fair value, Less than 12 months	126,139
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(1,861)
Estimated fair value, 12 months or more	841,748
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(187,710)
Privately issued commercial [Member]
Investment Securities Continuous Unrealized Loss Position Fair Value
Estimated fair value, Less than 12 months
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses
Estimated fair value, 12 months or more	12,127	15,029
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(921)	(2,650)
Collateralized Debt Obligations [Member]
Investment Securities Continuous Unrealized Loss Position Fair Value
Estimated fair value, Less than 12 months	3,106	2,871
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(39)	(335)
Estimated fair value, 12 months or more	5,349	4,863
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(1,691)	(2,153)
Other Debt Securities [Member]
Investment Securities Continuous Unrealized Loss Position Fair Value
Estimated fair value, Less than 12 months	16,858	72,637
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(2,053)	(9,883)
Estimated fair value, 12 months or more	74,206	73,635
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(29,205)	(34,560)
Equity Securities [Member]
Investment Securities Continuous Unrealized Loss Position Fair Value
Estimated fair value, Less than 12 months	8,118	9,883
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(1,389)	(4,500)
Estimated fair value, 12 months or more	2,193
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	$ (2,033)