Debt and Preferred Equity Investments - Additional Information (Details) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2014
Mortgage Loans on Real Estate [Line Items]
Aggregate weighted average current yield (as a percent)	9.06%
Debt Investments Held [Abstract]
Carrying Value	$ 2,168,515,000	$ 2,114,041,000
Mezzanine/Jr. Mortgage Loan with an Initial Maturity Date of April 2017
Debt Investments Held [Abstract]
Carrying Value	250,500,000	0	$ 0
Mezzanine Loan with an Initial Maturity Date of March 2020
Debt Investments Held [Abstract]
Amount participated out	1,300,000
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 2018
Debt Investments Held [Abstract]
Amount participated out	14,600,000
Mezzanine Loan with an Initial Maturity Date of December 2018, 2
Debt Investments Held [Abstract]
Amount participated out	14,100,000
Mezzanine Loan with an Initial Maturity of March 2019
Debt Investments Held [Abstract]
Amount participated out	5,100,000
Debt Investments in Mortgage Loans
Debt Investments Held [Abstract]
Future Funding Obligations	222,192,000
Senior Financing	7,150,615,000
Carrying Value	2,023,383,000	1,969,618,000
Total fixed rate
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	3,895,701,000
Carrying Value	424,938,000	669,968,000
Total fixed rate | Mezzanine/Jr. Mortgage Loan with an Initial Maturity Date of April 2017
Debt Investments Held [Abstract]
Interest Receivable	7,700,000
Total fixed rate | Mortgage Loan, February 2019
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	26,394,000	26,366,000
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	164,000	239,000
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	208,506,000	204,005,000
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	24,922,000	24,913,000
Total fixed rate | Mezzanine Loan, November 2023
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	86,201,000
Carrying Value	12,702,000	12,699,000
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	12,936,000	12,932,000
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	55,250,000
Total fixed rate | Real Estate Acquired in Satisfaction of Debt [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0	250,464,000
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	222,192,000
Senior Financing	3,254,914,000
Carrying Value	1,598,445,000	1,299,650,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of March 2020
Debt Investments Held [Abstract]
Future Funding Obligations	45,019,000
Senior Financing	0
Carrying Value	333,290,000	0
Total floating rate | Mortgage and Mezzanine Loan with an Initial Maturity Date of April 2020 [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	6,386,000
Senior Financing	35,912,000
Carrying Value	11,829,000	11,259,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	175,000,000
Carrying Value	37,355,000	37,250,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of April 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	61,130,000
Carrying Value	21,718,000	34,947,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of August 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	20,523,000
Carrying Value	10,970,000	10,934,000
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	19,990,000	19,940,000
Total floating rate | Mezzanine Loan With An Initial Maturity Date of August 2018, 2
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	65,000,000
Carrying Value	14,992,000	14,955,000
Total floating rate | Mortgage/Mezzanine Loan, September 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	16,992,000	16,969,000
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	14,961,000	14,855,000
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity Date of October 2016 [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	2,325,000
Senior Financing	45,025,000
Carrying Value	35,072,000	34,879,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of October 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	150,000,000
Carrying Value	15,347,000	15,381,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	14,836,000	14,869,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2018, 2
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	33,000,000
Carrying Value	26,967,000	26,927,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2018, 3
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	175,000,000
Carrying Value	59,873,000	59,723,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2018, 4
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	45,000,000
Carrying Value	12,211,000	12,174,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of January 2019
Debt Investments Held [Abstract]
Future Funding Obligations	4,000,000
Senior Financing	29,291,000
Carrying Value	9,980,000	8,550,000
Total floating rate | Mezzanine Loan with an Initial Maturity of January 2019, 2
Debt Investments Held [Abstract]
Future Funding Obligations	795,000
Senior Financing	0
Carrying Value	15,150,000	15,148,000
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	21,964,000	21,939,000
Total floating rate | Mezzanine Loan with an Initial Maturity of March 2019, 2
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	40,000,000
Carrying Value	19,958,000	19,982,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of April 2019
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	265,000,000
Carrying Value	24,895,000	24,830,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of April 2019, 2 [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	20,065,000
Senior Financing	220,572,000
Carrying Value	79,551,000	71,832,000
Total floating rate | Mortgage/Jr Mortgage Participate Loan, Maturity Date of August 2019
Debt Investments Held [Abstract]
Future Funding Obligations	2,034,000
Senior Financing	189,829,000
Carrying Value	37,884,000	37,851,000
Total floating rate | Mezzanine Loan, Maturity of September 2019
Debt Investments Held [Abstract]
Future Funding Obligations	20,560,000
Senior Financing	0
Carrying Value	168,369,000	143,919,000
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of September 2019
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	350,000,000
Carrying Value	34,810,000	34,737,000
Total floating rate | Mezzanine Loan with an Initial Maturity of October 2019 [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	1,306,000
Senior Financing	0
Carrying Value	46,168,000	0
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity Date of December 2019 [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	17,627,000
Senior Financing	0
Carrying Value	51,981,000	43,845,000
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of January 2020
Debt Investments Held [Abstract]
Future Funding Obligations	1,123,000
Senior Financing	571,863,000
Carrying Value	78,462,000	75,834,000
Total floating rate | Mezzanine Loan with an Initial Maturity of January 2020
Debt Investments Held [Abstract]
Future Funding Obligations	14,786,000
Senior Financing	0
Carrying Value	84,783,000	0
Total floating rate | Mortgage Loan with an Initial Maturity of February 2020 [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	3,878,000
Senior Financing	306,711,000
Carrying Value	50,669,000	0
Total floating rate | Mezzanine Loan with an Initial Maturity of July 2020 [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	44,088,000
Senior Financing	340,558,000
Carrying Value	83,065,000	75,428,000
Total floating rate | Mezzanine Loan with an Initial Maturity of July 2020 [Domain]
Debt Investments Held [Abstract]
Future Funding Obligations	38,200,000
Senior Financing	0
Carrying Value	93,498,000	88,989,000
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of December 2020 [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	35,207,000	35,152,000
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of December 2020, 2 [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	60,000,000
Carrying Value	15,648,000	15,635,000
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	0
Carrying Value	0	162,553,000
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	35,564,000	34,600,000
Total floating rate | Mortgage/Mezzanine Loan repaid in June 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	0	74,755,000
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	$ 23,609,000