Loans and Allowance for Losses and Concentrations of Credit Risk - Troubled Debt Restructurings (Details)	3 Months Ended	12 Months Ended
Sep. 30, 2017	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Receivables [Abstract]
Financing Receivable, Modifications, Pre-Modification Recorded Investment		$ 200,000	$ 0	$ 1,100,000
Financing Receivable, Modifications, Post-Modification Recorded Investment		$ 200,000	$ 0	$ 1,100,000
Financing Receivable, Modifications, Subsequent Default, Number of Contracts	0	0	0
Financing Receivable, Modifications, Subsequent Default, Recorded Investment		$ 0	$ 0
Financing Receivable, Modifications, Subsequent Default, Determination of Allowance for Credit Losses		0	0	0

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

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=84173941&loc=SL6953676-111524

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- Definition

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

+ References

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- Definition

Description of how troubled debt restructurings within the previous 12 months and for which there was a payment default during the period are factored into the determination of the allowance for credit losses.

+ 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=84173941&loc=SL6953682-111524

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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

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- Definition

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

+ References

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- References

No definition available.

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