Fair Value Disclosures - Estimated Fair Value and Related Carrying Amounts of Mortgage Loans and Bonds Payable (Details) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2020 USD ($) Loan	Dec. 31, 2019 USD ($) Contract
Liabilities:
Number of interest rate swap contracts \| Contract		2
Carrying Amount
Assets
Loans receivable	$ 51,984	$ 50,553
Liabilities:
Unsecured notes	2,373,767	1,985,603
Mortgage loans payable (fixed rate)	721,407	761,296
Bonds payable	23,033	23,001
Unsecured term loan (fixed rate)	199,297	199,121
Owned properties \| Mortgage loans payable
Liabilities:
Principal outstanding	$ 656,453	693,584
Owned properties \| Mortgage loans payable \| Variable rate mortgage loans
Liabilities:
Number of mortgage loans \| Loan	1
Principal outstanding	$ 2,600	3,100
Level 2 \| Estimated Fair Value
Assets
Loans receivable	0	0
Liabilities:
Unsecured notes	2,431,498	2,069,817
Mortgage loans payable (fixed rate)	770,468	766,821
Bonds payable	25,375	25,110
Unsecured term loan (fixed rate)	204,446	198,687
Level 3 \| Estimated Fair Value
Assets
Loans receivable	48,307	48,307
Liabilities:
Mortgage loans payable (fixed rate)	0	0
Bonds payable	0	0
Unsecured term loan (fixed rate)	$ 0	$ 0

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- Definition

Number Of Loans

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- Definition

Fair value portion of unsecured notes payable.

+ References

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- References

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

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-Section S99
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-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

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- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

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- Definition

Fair value portion of contractual obligation to pay money on demand or on fixed or determinable dates.

+ References

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Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

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- Definition

Fair value portion of notes payable.

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-Publisher FASB
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-Topic 210
-SubTopic 10
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-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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