Debt and Preferred Equity Investments - Debt Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	May 31, 2017	Dec. 31, 2016	Sep. 30, 2014
Mortgage Loans on Real Estate [Line Items]
Mortgage loans on real estate, interest rate	9.31%
Debt Investments Held [Abstract]
Carrying Value	$ 2,114,041		$ 1,640,412
Mezzanine Loan with an Initial Maturity Date of March 2020
Debt Investments Held [Abstract]
Amount participated out	1,200
Mezzanine Loan with an Initial Maturity Date of June 2024
Debt Investments Held [Abstract]
Amount participated out	12,000
Mezzanine Loan with an Initial Maturity of March 2018
Debt Investments Held [Abstract]
Amount participated out	5,100
Mezzanine Loan with an Initial Maturity Date of December 2018
Debt Investments Held [Abstract]
Amount participated out	14,600
Mezzanine Loan with an Initial Maturity Date of December 2018, 2
Debt Investments Held [Abstract]
Amount participated out	14,100
Mezzanine Loan with an Initial Maturity Date of January 2019
Debt Investments Held [Abstract]
Amount participated out	$ 21,200
Mezzanine Loan Contributed To Joint Venture In May 2017
Debt Investments Held [Abstract]
Equity Method Investment, Ownership Percentage	33.33%	33.33%
Debt investment
Debt Investments Held [Abstract]
Future Funding Obligations	$ 231,924
Senior Financing	7,140,238
Carrying Value	1,969,618		1,592,537
Total fixed rate
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	3,616,476
Carrying Value	635,368		360,359
Total fixed rate | Mortgage and Jr Mortgage Loan with an Initial Maturity Date of April 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	250,464		0
Loan acquired	259,300
Total fixed rate | Junior Mortgage Participation, Related To Mortgage Loan, February 2019
Debt Investments Held [Abstract]
Loan acquired				$ 5,700
Discount amount				5,700
Total fixed rate | Mortgage Loan With An Initial Maturity Date Of February 2019
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	26,366		26,311
Loan acquired				26,400
Discount amount				$ 200
Total fixed rate | Mortgage Loan with an Initial Maturity of August 2019
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	239		380
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
Carrying Value	204,005		0
Total fixed rate | Mezzanine Loan with an Initial Maturity Date of September 2021
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	15,000
Carrying Value	3,500		3,500
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
Carrying Value	24,913		0
Total fixed rate | Mezzanine Loan, with an Initial Maturity Date of November 2023
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	86,976
Carrying Value	12,699		12,692
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
Carrying Value	12,932		12,925
Total fixed rate | Mezzanine Loan, with an Initial Maturity Date of January 2025
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	95,000
Carrying Value	30,000		30,000
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
Carrying Value	15,000		15,000
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
Carrying Value	55,250		0
Total fixed rate | Mezzanine Loan Repaid in June 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0		66,129
Total fixed rate | Junior Mortgage Participation or Mezzanine Loan repaid in March 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0		193,422
Total floating rate
Debt Investments Held [Abstract]
Future Funding Obligations	231,924
Senior Financing	3,523,762
Carrying Value	1,334,250		1,232,178
Total floating rate | Mezzanine Loan with an Initial Maturity of March 2018
Debt Investments Held [Abstract]
Future Funding Obligations	795
Senior Financing	0
Carrying Value	15,148		15,051
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
Carrying Value	19,982		19,913
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	94,546
Carrying Value	34,947		34,844
Total floating rate | Mezzanine Loan with an Initial Maturity Date of August 2018
Debt Investments Held [Abstract]
Future Funding Obligations	523
Senior Financing	20,523
Carrying Value	10,934		10,863
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	19,940		19,840
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of August 2018, 2
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	65,000
Carrying Value	14,955		14,880
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity Date of September 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	16,969		16,960
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
Carrying Value	14,855		14,648
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of October 2018
Debt Investments Held [Abstract]
Future Funding Obligations	391
Senior Financing	0
Carrying Value	23,609		20,423
Total floating rate | Mezzanine Loan with an Initial Maturity Date of October 2018
Debt Investments Held [Abstract]
Future Funding Obligations	2,325
Senior Financing	45,025
Carrying Value	34,879		34,502
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
Carrying Value	74,755		74,476
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	85,000
Carrying Value	15,381		15,141
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2018, 2
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	65,000
Carrying Value	14,869		14,656
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2018, 3
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	33,000
Carrying Value	26,927		26,850
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2018, 4
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	175,000
Carrying Value	59,723		56,114
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
Carrying Value	12,174		12,104
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity Date of January 2019
Debt Investments Held [Abstract]
Future Funding Obligations	26,284
Senior Financing	0
Carrying Value	162,553		0
Total floating rate | Mezzanine Loan with an Initial Maturity of January 2019, 3
Debt Investments Held [Abstract]
Future Funding Obligations	5,372
Senior Financing	25,289
Carrying Value	8,550		5,410
Total floating rate | Mezzanine Loan with an Initial Maturity of March 2019
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	38,000
Carrying Value	21,939		21,891
Total floating rate | Mezzanine Loan with an Initial Maturity Date of April 2019
Debt Investments Held [Abstract]
Future Funding Obligations	11
Senior Financing	174,947
Carrying Value	37,250		0
Total floating rate | Mezzanine Loan with an Initial Maturity Date of April 2019, 2
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	265,000
Carrying Value	24,830		24,707
Total floating rate | Mortgage/Jr Mortgage Participate Loan, Maturity Date of August 2019
Debt Investments Held [Abstract]
Future Funding Obligations	27,617
Senior Financing	199,733
Carrying Value	71,832		65,554
Total floating rate | Mezzanine Loan, Maturity of September 2019
Debt Investments Held [Abstract]
Future Funding Obligations	2,034
Senior Financing	189,829
Carrying Value	37,851		37,322
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of September 2019
Debt Investments Held [Abstract]
Future Funding Obligations	36,391
Senior Financing	0
Carrying Value	143,919		111,819
Total floating rate | Mezzanine Loan with an Initial Maturity of October 2019
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	350,000
Carrying Value	34,737		0
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of January 2020
Debt Investments Held [Abstract]
Future Funding Obligations	25,643
Senior Financing	0
Carrying Value	43,845		33,682
Total floating rate | Mezzanine Loan with an Initial Maturity of January 2020
Debt Investments Held [Abstract]
Future Funding Obligations	3,621
Senior Financing	555,379
Carrying Value	75,834		125,911
Amount participated out	66,100
Total floating rate | Mezzanine Loan with an Initial Maturity of July 2020
Debt Investments Held [Abstract]
Future Funding Obligations	6,913
Senior Financing	34,337
Carrying Value	11,259		0
Total floating rate | Mezzanine Loan with an Initial Maturity of November 2020
Debt Investments Held [Abstract]
Future Funding Obligations	51,494
Senior Financing	310,654
Carrying Value	75,428		63,137
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of December 2020
Debt Investments Held [Abstract]
Future Funding Obligations	42,510
Senior Financing	0
Carrying Value	88,989		0
Total floating rate | Mortgage/Mezzanine Loan 2 with an Initial Maturity of December 2020
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	35,152		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
Carrying Value	15,635		15,606
Total floating rate | Mezzanine Loan With An Initial Maturity Of August 2022
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	280,000
Carrying Value	34,600		0
Total floating rate | Mortgage/Mezzanine Loan Repaid in December 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0		29,998
Total floating rate | Mezzanine Loan Repaid In November 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0		64,505
Total floating rate | Mezzanine Loan, repaid in September 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0		15,369
Total floating rate | Mortgage/Mezzanine Loan repaid in June 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0		32,847
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		22,959
Total floating rate | Mezzanine Loan Contributed To Joint Venture In May 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0		14,957
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