Debt and Preferred Equity Investments - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2017	Feb. 28, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2014
Mortgage Loans on Real Estate [Line Items]
Aggregate weighted average current yield (as a percent)	9.49%
Debt Investments Held [Abstract]
Carrying Value	$ 2,020,739,000		$ 1,640,412,000
Mezzanine/Jr. Mortgage Loan with an Initial Maturity Date of April 2017
Debt Investments Held [Abstract]
Carrying Value	250,200,000		0	$ 0
Mezzanine Loan with an Initial Maturity Date of March 2020
Debt Investments Held [Abstract]
Amount participated out	1,200,000
Mezzanine Loan, with an Initial Maturity Date of June 2024
Debt Investments Held [Abstract]
Amount participated out	12,000,000
Mezzanine Loan with an Initial Maturity Date of December 2017
Debt Investments Held [Abstract]
Amount participated out	14,600,000
Mezzanine Loan with an Initial Maturity Date of December 2017, 2
Debt Investments Held [Abstract]
Amount participated out	14,100,000
Mortgage Mezzanine Loan with an Initial Maturity Date of December 2017
Debt Investments Held [Abstract]
Amount participated out	5,100,000
Mortgage/Mezzanine Loan with an Initial Maturity Date of January 2019
Debt Investments Held [Abstract]
Amount participated out	21,200,000
Debt Investments in Mortgage Loans
Debt Investments Held [Abstract]
Future Funding Obligations	181,320,000
Senior Financing	7,115,310,000
Carrying Value	1,876,759,000		1,592,537,000
Total fixed rate
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	3,617,095,000
Carrying Value	632,834,000		360,359,000
Total fixed rate | Mezzanine/Jr. Mortgage Loan with an Initial Maturity Date of April 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	250,164,000		0
Loan acquired	259,300,000
Total fixed rate | Mortgage Loan, February 2019
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	26,352,000		26,311,000
Loan acquired					$ 26,400,000
Discount amount					200,000
Total fixed rate | Mortgage Loan, August 2019
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	275,000		380,000
Total fixed rate | Mezzanine Loan with an Initial Maturity Date of March 2020
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	1,160,000,000
Carrying Value	201,757,000		0
Total fixed rate | Mezzanine Loan, September 2021
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	15,000,000
Carrying Value	3,500,000		3,500,000
Total fixed rate | Mezzanine Loan with an Initial Maturity Date of April 2022
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	147,000,000
Carrying Value	24,909,000		0
Total fixed rate | Mezzanine Loan, November 2023
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	87,595,000
Carrying Value	12,697,000		12,692,000
Total fixed rate | Mezzanine Loan, with an Initial Maturity Date of June 2024
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	115,000,000
Carrying Value	12,930,000		12,925,000
Total fixed rate | Mezzanine Loan, January 2025
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	95,000,000
Carrying Value	30,000,000		30,000,000
Total fixed rate | Mezzanine Loan with an Initial Maturity Date of November 2026
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	340,000,000
Carrying Value	15,000,000		15,000,000
Total fixed rate | Mezzanine Loan with an Initial Maturity Date of June 2027
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	1,657,500,000
Carrying Value	55,250,000		0
Total fixed rate | Mezzanine Loan, June 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0		66,129,000
Total fixed rate | Junior Mortgage Participation/Mezzanine Loan, March 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0		193,422,000
Total fixed rate | Junior Mortgage Participation, Related To Mortgage Loan, February 2019
Debt Investments Held [Abstract]
Loan acquired					5,700,000
Discount amount					$ 5,700,000
Total floating rate
Debt Investments Held [Abstract]
Future Funding Obligations	181,320,000
Senior Financing	3,498,215,000
Carrying Value	1,243,925,000		1,232,178,000
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity Date of October 2017
Debt Investments Held [Abstract]
Future Funding Obligations	622,000
Senior Financing	0
Carrying Value	23,372,000		20,423,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	85,000,000
Carrying Value	15,340,000		15,141,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2017, 2
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	65,000,000
Carrying Value	14,832,000		14,656,000
Total floating rate | Mortgage Mezzanine Loan with an Initial Maturity Date of December 2017
Debt Investments Held [Abstract]
Future Funding Obligations	795,000
Senior Financing	0
Carrying Value	15,132,000		15,051,000
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity Date of January 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	125,000,000
Carrying Value	29,966,000		29,998,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of April 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	40,000,000
Carrying Value	19,964,000		19,913,000
Total floating rate | Junior Mortgage Participation Loan with an Initial Maturity Date of April 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	117,808,000
Carrying Value	34,899,000		34,844,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of August 2018
Debt Investments Held [Abstract]
Future Funding Obligations	523,000
Senior Financing	20,523,000
Carrying Value	10,916,000		10,863,000
Total floating rate | Mortgage/Mezzanine Loan, with an Initial Maturity Date of August 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	19,914,000		19,840,000
Total floating rate | Mortgage Loan with an Initial Maturity Date of August 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	65,000,000
Carrying Value	14,935,000		14,880,000
Total floating rate | Mortgage/Mezzanine Loan, September 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	16,957,000		16,960,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of September 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	37,500,000
Carrying Value	14,801,000		14,648,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of October 2018
Debt Investments Held [Abstract]
Future Funding Obligations	2,325,000
Senior Financing	45,025,000
Carrying Value	34,782,000		34,502,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of November 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	335,000,000
Carrying Value	74,683,000		74,476,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	33,000,000
Carrying Value	26,907,000		26,850,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2018, 2
Debt Investments Held [Abstract]
Future Funding Obligations	1,050,000
Senior Financing	171,939,000
Carrying Value	58,598,000		56,114,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2018, 3
Debt Investments Held [Abstract]
Future Funding Obligations	8,267,000
Senior Financing	289,621,000
Carrying Value	71,067,000		63,137,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2018, 4
Debt Investments Held [Abstract]
Future Funding Obligations	5,197,000
Senior Financing	229,084,000
Carrying Value	74,314,000		64,505,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of January 2019
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	45,000,000
Carrying Value	12,156,000		12,104,000
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity Date of January 2019
Debt Investments Held [Abstract]
Future Funding Obligations	30,101,000
Senior Financing	0
Carrying Value	158,757,000		0
Total floating rate | Mezzanine Loan with an Initial Maturity of January 2019, 2
Debt Investments Held [Abstract]
Future Funding Obligations	6,081,000
Senior Financing	24,086,000
Carrying Value	7,812,000		5,410,000
Total floating rate | Mezzanine Loan with an Initial Maturity of March 2019
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	38,000,000
Carrying Value	21,927,000		21,891,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of April 2019
Debt Investments Held [Abstract]
Future Funding Obligations	279,000
Senior Financing	173,700,000
Carrying Value	36,936,000		0
Total floating rate | Mezzanine Loan with an Initial Maturity Date of April 2019, 2 [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	265,000,000
Carrying Value	24,798,000		24,707,000
Total floating rate | Mortgage/Jr Mortgage Participate Loan, Maturity Date of August 2019
Debt Investments Held [Abstract]
Future Funding Obligations	29,661,000
Senior Financing	194,094,000
Carrying Value	69,705,000		65,554,000
Total floating rate | Mezzanine Loan, Maturity of September 2019
Debt Investments Held [Abstract]
Future Funding Obligations	2,034,000
Senior Financing	187,500,000
Carrying Value	37,835,000		37,322,000
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of September 2019
Debt Investments Held [Abstract]
Future Funding Obligations	49,933,000
Senior Financing	0
Carrying Value	130,350,000		111,819,000
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of January 2020
Debt Investments Held [Abstract]
Future Funding Obligations	30,494,000
Senior Financing	0
Carrying Value	38,934,000		33,682,000
Total floating rate | Mezzanine Loan with an Initial Maturity of January 2020
Debt Investments Held [Abstract]
Future Funding Obligations	6,794,000
Senior Financing	537,748,000
Carrying Value	72,597,000	$ 66,100,000	125,911,000
Total floating rate | Mezzanine Loan with an Initial Maturity of July 2020 [Domain]
Debt Investments Held [Abstract]
Future Funding Obligations	7,164,000
Senior Financing	33,587,000
Carrying Value	10,988,000		0
Total floating rate | Jr Mortgage Participation/Mezzanine Loan with an Initial Maturity of July 2021
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	60,000,000
Carrying Value	15,627,000		15,606,000
Total floating rate | Mezzanine Loan with an Initial Maturity of August 2022
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	280,000,000
Carrying Value	34,124,000		0
Total floating rate | Mezzanine Loan, with an Initial Maturity Date of September 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0		15,369,000
Total floating rate | Mortgage/Mezzanine Loan repaid in June 2017, 2
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0		32,847,000
Total floating rate | Mortgage/Mezzanine Loan repaid in June 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0		22,959,000
Total floating rate | Mezzanine Loan Contributed for a Joint Venture Interest
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0		14,957,000
Total floating rate | Mortgage/Mezzanine Loan Repaid in January 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	$ 0		$ 145,239,000