Borrowings - Mortgage Loan Participation and Sale Agreement (Details) - USD ($)	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
During the period:
Total interest expense	$ 2,533,000	$ 1,484,000	$ 6,450,000	$ 3,780,000
Carrying value:
Mortgage loan participation and sale agreement secured by mortgage loan participation certificates	524,667,000		524,667,000		$ 527,395,000
Mortgage Loan Participation and Sale Agreement member
During the period:
Average balance	$ 289,008,000	$ 213,486,000	$ 250,599,000	$ 200,119,000
Weighted-average interest rate (as a percent)	3.31%	2.48%	3.14%	2.25%
Total interest expense	$ 2,533,000	$ 1,484,000	$ 6,450,000	$ 3,780,000
Carrying value:
Unpaid principal balance of mortgage loan participation and sale agreement secured by mortgage loan participation certificates	524,686,000		524,686,000		527,706,000
Unamortized issuance costs	(19,000)		(19,000)		(311,000)
Mortgage loan participation and sale agreement secured by mortgage loan participation certificates	$ 524,667,000		$ 524,667,000		$ 527,395,000
Weighted-average interest rate (as a percent)	3.51%		3.51%		2.81%
Fair value of mortgage loans pledged to secure	$ 547,969,000		$ 547,969,000		$ 551,688,000
Amortization of debt issuance costs	92,000	134,000	475,000	365,000
Maximum \| Mortgage Loan Participation and Sale Agreement member
Carrying value:
Mortgage loan participation and sale agreement secured by mortgage loan participation certificates		532,266,000		532,266,000
Mortgage loan participation and sale agreement secured by mortgage loan participation certificates	$ 722,611,000	$ 532,266,000	$ 722,611,000	$ 532,266,000

Represents the interest expense related to mortgage loans participation and sale agreement.

No definition available.

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- Definition

Participation certificates in mortgage loans that have been pooled with Fannie Mae, Freddie Mac or Ginnie Mae, are issued to a lender pending the securitization of the mortgage loans and sale of the resulting securities.

+ References

No definition available.

+ Details

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- Definition

Amount of unpaid principal balance of mortgage loan participation and sale agreement

+ References

No definition available.

+ Details

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- Definition

+ References

No definition available.

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- References

No definition available.

+ Details

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Balance Type:	na
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- References

No definition available.

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- Definition

Amount of amortization expense attributable to debt issuance costs.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(8))
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28555-108399

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Period Type:	duration

- Definition

The average effective interest rate during the reporting period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

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- Definition

The sum of the fair values, as of the date of each statement of financial position presented, of securities that are restricted as collateral for the payment of related debt obligations, primarily secured borrowings and repurchase agreements, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

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- Definition

For the form of debt having an initial term of less than one year or less than the normal operating cycle, if longer, average borrowings during the period.

+ References

No definition available.

+ Details

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- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Details

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