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Fair Value Measurements - Interest Rate Derivatives (Details) - Interest rate derivatives - Cash flow hedges	3 Months Ended
	Mar. 31, 2018 USD ($) item
Fair Value Measurements
Number of interest rate derivatives held	28
Number of mortgage loans outstanding	28
Number of mortgage loans designated as cash flow hedges	28
Notional Amount | $	$ 132,039,000
Weighted average maturity	6 years 9 months 18 days
Weighted average annual interest rate (as a percent)	4.11%
Minimum
Fair Value Measurements
Fixed Interest Rate (as a percent)	3.02%
Maximum
Fair Value Measurements
Fixed Interest Rate (as a percent)	5.38%

X

- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

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X

- Definition

Represents the number of mortgage loans outstanding related to derivative instruments at the balance sheet date.

+ References

No definition available.

+ Details

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- Definition

Represents the number of mortgage loans designated as cash flow hedges.

+ References

No definition available.

+ Details

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X

- Definition

Average fixed interest rate related to the group of interest rate derivatives.

+ References

No definition available.

+ Details

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X

- Definition

Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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X

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

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X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

The number of derivative instruments of a particular group held by the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5579245-113959

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