Loans and Allowance for Losses and Concentrations of Credit Risk - Troubled Debt Restructurings (Details) (USD $)	12 Months Ended
Dec. 31, 2014 defaulted_loan	Dec. 31, 2013 defaulted_loan	Dec. 31, 2012
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 5,300,000	$ 1,100,000	$ 2,600,000
Financing Receivable, Modifications, Post-Modification Recorded Investment	6,000,000	1,100,000	2,800,000
Financing Receivable, Modifications, Subsequent Default, Number of Contracts	0	0
Financing Receivable, Modifications, Subsequent Default, Recorded Investment	0	0
Total (release of)/provision for losses	(3,166,000)	448,000	1,875,000
Loans Collateralized by Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total (release of)/provision for losses	$ 0	$ 100,000	$ (300,000)

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Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 33

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524

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- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

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- Definition

Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 34

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953682-111524

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Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default.

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- Definition

Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 11

-Article 9

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-Name Accounting Standards Codification

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