Debt and Preferred Equity Investments - Narrative (Details)	9 Months Ended	37 Months Ended
	Sep. 30, 2017 USD ($) segment	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Preferred equity investment
Carrying Value	$ 2,020,739,000	$ 2,020,739,000	$ 1,640,412,000
Number of portfolio segments of financial receivables (segment) | segment	1
Additional amount of financing receivables included in other assets	$ 65,600,000	65,600,000	99,500,000
Mezzanine/Jr. Mortgage Loan with an Initial Maturity Date of April 2017
Preferred equity investment
Carrying Value	250,200,000	250,200,000	0	$ 0
Junior Mortgage Participation Acquired in September 2014
Preferred equity investment
Payments to acquire loans		0
Carrying Value	0	0	$ 0	$ 0
Total fixed rate | Mezzanine/Jr. Mortgage Loan with an Initial Maturity Date of April 2017
Preferred equity investment
Receivable with Imputed Interest, Face Amount	$ 259,300,000	$ 259,300,000