Investment securities and impairment of investment securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Amortized cost
Total	$ 906,028	$ 906,702
Gross unrealized holding gains
Total	14,837	14,404
Gross unrealized holding losses
Total	(4,442)	(8,735)
Fair value
Total Fair value	916,423	912,371
U.S. government and agencies
Amortized cost
Due in one year or less	23	25
Fair value
Due in one year or less	23	25
Debt issued by government sponsored enterprises
Amortized cost
Due in one year - five years	339,140	310,172
Due in five years - ten years	25,128	25,746
Gross unrealized holding gains
Due in one year - five years	706	287
Gross unrealized holding losses
Due in one year - five years	(693)	(2,672)
Due in five years - ten years	(29)	(28)
Fair value
Due in one year - five years	339,153	307,787
Due in five years - ten years	25,099	25,718
Equity securities
Amortized cost
Total	2,314	2,591
Gross unrealized holding gains
Total	651	682
Gross unrealized holding losses
Total	(41)	(116)
Fair value
Total Fair value	2,924	3,157
Municipal securities
Amortized cost
Due in one year or less	1,159	810
Due in one year - five years	7,755	7,878
Due in five years - ten years	5,564	6,965
Due after ten years	50,236	51,839
Gross unrealized holding gains
Due in one year or less	10	15
Due in one year - five years	159	132
Due in five years - ten years	104	115
Due after ten years	2,292	2,391
Fair value
Due in one year or less	1,169	825
Due in one year - five years	7,914	8,010
Due in five years - ten years	5,668	7,080
Due after ten years	52,528	54,230
Corporate debt issues
Amortized cost
Due after ten years	18,080	18,267
Gross unrealized holding gains
Due after ten years	2,646	2,579
Gross unrealized holding losses
Due after ten years	(444)	(419)
Fair value
Due after ten years	20,282	20,427
Residential mortgage-backed securities
Amortized cost
Total	456,629	482,409
Gross unrealized holding gains
Total	8,269	8,203
Gross unrealized holding losses
Total	(3,235)	(5,500)
Fair value
Total Fair value	461,663	485,112
Fixed rate | Pass-through
Amortized cost
Total	69,549	72,852
Gross unrealized holding gains
Total	3,423	3,149
Gross unrealized holding losses
Total	(130)	(124)
Fair value
Total Fair value	72,842	75,877
Fixed rate | Non-agency CMOs
Amortized cost
Total	2,978	3,162
Gross unrealized holding gains
Total	335	246
Fair value
Total Fair value	3,313	3,408
Fixed rate | Agency CMOs
Amortized cost
Total	214,343	226,413
Gross unrealized holding gains
Total	709	685
Gross unrealized holding losses
Total	(3,070)	(5,331)
Fair value
Total Fair value	211,982	221,767
Variable rate | Pass-through
Amortized cost
Total	62,918	66,140
Gross unrealized holding gains
Total	3,180	3,466
Gross unrealized holding losses
Total	(8)	(8)
Fair value
Total Fair value	66,090	69,598
Variable rate | Agency CMOs
Amortized cost
Total	106,841	113,842
Gross unrealized holding gains
Total	622	657
Gross unrealized holding losses
Total	(27)	(37)
Fair value
Total Fair value	$ 107,436	$ 114,462