Debt and Preferred Equity Investments - Rollforward of Net Book Balance (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 1,640,412	$ 1,670,020
Redemptions/Sales/Syndications/Amortization	(708,180)	(1,044,482)
Carrying Value	2,020,739	1,640,412
Debt Investments in Mortgage Loans
Mortgage Loans on Real Estate [Line Items]
Originations/Accretion	944,494	1,009,176
Preferred equity investments
Mortgage Loans on Real Estate [Line Items]
Originations/Accretion	$ 144,013	$ 5,698

Total carrying amount of mortgage loans as of the balance sheet date.

Reference 1: 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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29.6)
-URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014

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Amount of decrease in mortgage loans on real estate from amortization of premium.

+ References

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Total amount of new mortgage loans on real estate during the reporting period.

+ References

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 210.12-29.6)
-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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