Summary of Significant Accounting Policies - Mezzanine Loans (Details) - Mezzanine Loans	12 Months Ended
Dec. 31, 2017
Minimum
Mortgage loans on real estate
Interest rate for mortgage loan (as a percent)	12.00%
Loan Term	4 years
Maximum
Mortgage loans on real estate
Interest rate for mortgage loan (as a percent)	18.00%
Loan Term	8 years

The stated interest rate on the mortgage loan receivable or the weighted average interest rate on a group of loans.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29)
-URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 10
-Section S99
-Paragraph 6
-Subparagraph (SX 210.5-04.(c) Schedule IV)
-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

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