Loans and allowance for credit losses (Details 7) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Allowance for credit losses:
Beginning balance	$ 15,723	$ 12,990	$ 15,991	$ 14,288
Charge-offs	(27,019)	(6,391)	(32,649)	(19,902)
Recoveries	137	156	649	474
Net charge-offs	(26,882)	(6,235)	(32,000)	(19,428)
Provision	22,460	6,200	27,310	18,095
Ending balance	11,301	12,955	11,301	12,955
Unallocated Financing Receivables [Member]
Allowance for credit losses:
Beginning balance	87	408	187	22
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Net charge-offs	0	0	0	0
Provision	384	(408)	284	(22)
Ending balance	471	0	471	0
Commercial Loan [Member]
Allowance for credit losses:
Beginning balance	1,216	1,063	1,682	1,498
Charge-offs	(196)	(73)	(419)	(4,513)
Recoveries	47	22	149	230
Net charge-offs	(149)	(51)	(270)	(4,283)
Provision	205	524	(140)	4,321
Ending balance	1,272	1,536	1,272	1,536
Consumer Loan [Member]
Allowance for credit losses:
Beginning balance	301	304	407	594
Charge-offs	(3)	(31)	(65)	(208)
Recoveries	16	8	33	17
Net charge-offs	13	(23)	(32)	(191)
Provision	(44)	(158)	(105)	(99)
Ending balance	270	304	270	304
Residential Real Estate [Member]
Allowance for credit losses:
Beginning balance	4,259	4,237	5,194	5,014
Charge-offs	(4,225)	(2,323)	(6,114)	(7,996)
Recoveries	51	5	349	99
Net charge-offs	(4,174)	(2,318)	(5,765)	(7,897)
Provision	3,645	2,268	4,301	7,070
Ending balance	3,730	4,187	3,730	4,187
Commercial Real Estate [Member]
Allowance for credit losses:
Beginning balance	5,514	3,952	4,134	3,415
Charge-offs	(4,056)	(742)	(6,003)	(2,177)
Recoveries	20	116	113	123
Net charge-offs	(4,036)	(626)	(5,890)	(2,054)
Provision	2,090	275	5,324	2,240
Ending balance	3,568	3,601	3,568	3,601
Construction Loans [Member]
Allowance for credit losses:
Beginning balance	4,346	2,845	4,387	3,745
Charge-offs	(18,539)	(3,222)	(20,048)	(5,008)
Recoveries	3	5	5	5
Net charge-offs	(18,536)	(3,217)	(20,043)	(5,003)
Provision	16,180	3,699	17,646	4,585
Ending balance	$ 1,990	$ 3,327	$ 1,990	$ 3,327