Debt and Preferred Equity Investments - Debt Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgage loans on real estate, interest rate	8.99%
Debt Investments Held [Abstract]
Carrying value	$ 2,099,393	$ 2,114,041
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 2019
Debt Investments Held [Abstract]
Amount participated out	14,600
Mezzanine Loan With An Initial Maturity Date Of March 2019, 3
Debt Investments Held [Abstract]
Amount participated out	14,100
Mezzanine Loan with an Initial Maturity Date of March 2020
Debt Investments Held [Abstract]
Amount participated out	1,300
Debt investment
Debt Investments Held [Abstract]
Future Funding Obligations	156,293
Senior Financing	6,713,884
Carrying value	1,812,936	1,969,618
Total fixed rate
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	4,129,847
Carrying Value	430,099	669,968
Total fixed rate | Mezzanine Loan With An Initial Maturity Of August 2022
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	280,000
Carrying Value	36,585	34,600
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	24,932	24,913
Total fixed rate | Mezzanine Loan, with an Initial Maturity Date of November 2023
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	85,097
Carrying Value	12,706	12,699
Total fixed rate | Mezzanine Loan With An Initial Maturity Date Of December 2023
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	180,000
Carrying Value	30,000	0
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	12,941	12,932
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	11,000	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	1,712,750
Carrying Value	55,250	55,250
Total fixed rate | Mortgage and Jr Mortgage loan satisfied via foreclosure
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0	250,464
Total fixed rate | Mezzanine Loan Repaid In August 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0	26,366
Total fixed rate | Mortgage Loan Repaid In September 2018 [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0	239
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	213,185	204,005
Total floating rate
Debt Investments Held [Abstract]
Future Funding Obligations	156,293
Senior Financing	2,584,037
Carrying value	1,382,837	1,299,650
Total floating rate | Mezzanine Loan with an Initial Maturity Date of January 2019
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	45,025
Carrying value	37,499	34,879
Total floating rate | Mezzanine Loan with an Initial Maturity of March 2019
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	85,000
Carrying value	15,333	15,381
Total floating rate | Mezzanine Loan With An Initial Maturity Date Of March 2019, 3
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	65,000
Carrying value	14,822	14,869
Total floating rate | Mezzanine Loan with an Initial Maturity of March 2019, 2
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	38,000
Carrying value	21,990	21,939
Total floating rate | Mezzanine Loan with an Initial Maturity Date of April 2019
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	40,000
Carrying value	19,986	19,982
Total floating rate | Mezzanine Loan With An Initial Maturity Date Of April 2019, 4
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	265,000
Carrying value	24,961	24,830
Total floating rate | Mortgage/Jr Mortgage Participate Loan, Maturity Date of August 2019
Debt Investments Held [Abstract]
Future Funding Obligations	40,530
Senior Financing	233,086
Carrying value	84,012	71,832
Total floating rate | Mezzanine Loan With An Initial Maturity Date Of August 2019
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	65,000
Carrying value	14,998	14,955
Total floating rate | Mortgage/Mezzanine Loan With An Initial Maturity Date Of August 2019
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying value	19,999	19,940
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of September 2019
Debt Investments Held [Abstract]
Future Funding Obligations	1,027
Senior Financing	0
Carrying value	154,070	143,919
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	34,886	34,737
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of January 2020
Debt Investments Held [Abstract]
Future Funding Obligations	7,243
Senior Financing	0
Carrying value	62,493	43,845
Total floating rate | Mezzanine Loan with an Initial Maturity of January 2020
Debt Investments Held [Abstract]
Future Funding Obligations	559
Senior Financing	575,955
Carrying value	79,164	75,834
Total floating rate | Mortgage Loan with an Initial Maturity of February 2020
Debt Investments Held [Abstract]
Future Funding Obligations	11,204
Senior Financing	0
Carrying value	88,501	0
Total floating rate | Mezzanine Loan with an Initial Maturity Date of March 2020
Debt Investments Held [Abstract]
Future Funding Obligations	1,277
Senior Financing	322,300
Carrying value	53,402	0
Total floating rate | Mortgage and Mezzanine Loan with an Initial Maturity Date of April 2020
Debt Investments Held [Abstract]
Future Funding Obligations	14,860
Senior Financing	0
Carrying value	277,694	0
Total floating rate | Mortgage And Mezzanine Loan With An Initial Maturity Date Of June 2020
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying value	37,094	0
Total floating rate | Mezzanine Loan with an Initial Maturity of July 2020
Debt Investments Held [Abstract]
Future Funding Obligations	7,887
Senior Financing	38,167
Carrying value	12,627	11,259
Total floating rate | Mortgage And Mezzanine Loan With An Initial Maturity Date Of October 2020
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying value	83,449	0
Total floating rate | Mezzanine Loan with an Initial Maturity of November 2020
Debt Investments Held [Abstract]
Future Funding Obligations	38,575
Senior Financing	362,908
Carrying value	88,817	75,428
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of December 2020
Debt Investments Held [Abstract]
Future Funding Obligations	33,131
Senior Financing	0
Carrying value	98,804	88,989
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of December 2020, 2
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying value	35,266	35,152
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	15,665	15,635
Total floating rate | Mezzanine Loan With An Initial Maturity Date Of December 2021
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	38,596
Carrying value	7,305	34,947
Total floating rate | Mortgage/Mezzanine Loan Repaid In February 2018, 2
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying value	0	162,553
Total floating rate | Mortgage/Mezzanine Loan repaid in February 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying value	0	74,755
Total floating rate | Mezzanine Loan Sold In 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying value	0	23,609
Total floating rate | Mortgage And Mezzanine Loan Repaid In 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying value	0	16,969
Total floating rate | Mezzanine Loan Repaid In 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying value	0	59,723
Total floating rate | Mezzanine Loan Repaid In 2018, 2
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying value	0	37,851
Total floating rate | Mezzanine Loan Repaid In 2018, 3
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying value	0	14,855
Total floating rate | Mezzanine Loan Exchanged For Property In 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying value	0	12,174
Total floating rate | Mezzanine Loan Exchanged For Property In 2018, 2
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying value	0	10,934
Total floating rate | Mezzanine Loan Repaid In 2018, 4
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying value	0	37,250
Total floating rate | Mezzanine Loan Repaid In 2018, 5
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying value	0	15,148
Total floating rate | Mezzanine Loan Repaid In 2018, 6
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying value	0	8,550
Total floating rate | Mezzanine Loan Exchanged For Property In 2018, 3
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying value	$ 0	$ 26,927