Loans Receivable and Other Lending Investments, net (Credit Characteristics by Payment Status) (Details) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018 USD ($) loan	Dec. 31, 2017 USD ($) loan
Recorded investment in loans, aged by payment status and presented by class
Current	$ 1,217,138	$ 1,062,944
Less Than and Equal to 90 Days	0	0
Greater Than 90 Days	218,113	231,877
Total Past Due	218,113	231,877
Loans	$ 1,435,251	$ 1,294,821
Financing receivable, number of loans greater than 90 days past due \| loan	3	4
Financing receivables, past due time period	90 days	90 days
Minimum
Recorded investment in loans, aged by payment status and presented by class
Financing receivables, past due time period	2 years	1 year
Maximum
Recorded investment in loans, aged by payment status and presented by class
Financing receivables, past due time period	9 years	9 years
Senior mortgages
Recorded investment in loans, aged by payment status and presented by class
Current	$ 848,509	$ 719,057
Less Than and Equal to 90 Days	0	0
Greater Than 90 Days	61,437	75,343
Total Past Due	61,437	75,343
Loans	909,946	794,400
Corporate/Partnership loans
Recorded investment in loans, aged by payment status and presented by class
Current	358,943	334,364
Less Than and Equal to 90 Days	0	0
Greater Than 90 Days	156,676	156,534
Total Past Due	156,676	156,534
Loans	515,619	490,898
Subordinate mortgages
Recorded investment in loans, aged by payment status and presented by class
Current	9,686	9,523
Less Than and Equal to 90 Days	0	0
Greater Than 90 Days	0	0
Total Past Due	0	0
Loans	$ 9,686	$ 9,523

Financing Receivable, Number of Loans Greater than 90 Days Past Due

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- Definition

Financing receivables that are equal to or less than 90 days past due.

+ References

No definition available.

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- Definition

Financing receivables that are greater than 90 days past due.

+ References

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- Definition

Financing Receivables, Past Due Time Period

+ References

No definition available.

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- Definition

Financing receivables that are current.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953401-111524

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- Definition

Recorded Investment in financing receivables that are past due at the balance sheet date.

+ References

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- 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.

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- Definition

Amount representing an agreement for an unconditional promise by the maker to pay the entity (holder) a definite sum of money at a future date. Such amount may include accrued interest receivable in accordance with the terms of the note. The note also may contain provisions and related items including a discount or premium, payable on demand, secured, or unsecured, interest bearing or noninterest bearing, among a myriad of other features and characteristics. Excludes amounts related to receivables held-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10(3))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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