Commitments, guarantees and contingencies (Narrative) (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet financial instruments	$ 404,894,000	$ 272,388,000
Operating lease rent expense	3,000,000us-gaap_OperatingLeasesRentExpenseNet	2,100,000us-gaap_OperatingLeasesRentExpenseNet	2,100,000us-gaap_OperatingLeasesRentExpenseNet
Pledged Letter Of Credit Issued By FHLB Collateralizing Public Deposits
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Guarantor obligations current carrying value	$ 0

The fair value of financial liabilities, which are not recognized in the financial statements (off-balance sheet) because they fail to meet some other criterion for recognition.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611

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- Definition

The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 460

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

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- Definition

Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41499-112717

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