Debt and Preferred Equity Investments - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 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)	9.19%
Debt Investments Held [Abstract]
Carrying Value	$ 2,085,871,000	$ 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	199,445,000
Senior Financing	7,120,078,000
Carrying Value	1,941,045,000	1,969,618,000
Total fixed rate
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	3,896,080,000
Carrying Value	672,652,000	669,968,000
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	250,464,000	250,464,000
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	26,380,000	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	201,000	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	206,230,000	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	24,918,000	24,913,000
Total fixed rate | Mezzanine Loan, November 2023
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	86,580,000
Carrying Value	12,701,000	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	12,934,000	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 | 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	199,445,000
Senior Financing	3,223,998,000
Carrying Value	1,268,393,000	1,299,650,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of March 2020
Debt Investments Held [Abstract]
Future Funding Obligations	5,993,000
Senior Financing	294,036,000
Carrying Value	48,490,000	0
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2017
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	75,014,000
Carrying Value	23,346,000	34,947,000
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	19,999,000	19,982,000
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	10,952,000	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	19,965,000	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	14,973,000	14,955,000
Total floating rate | Mortgage/Mezzanine Loan, September 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	0
Carrying Value	16,980,000	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	14,908,000	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	391,000
Senior Financing	0
Carrying Value	23,609,000	23,609,000
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	34,975,000	34,879,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2018
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	85,000,000
Carrying Value	15,327,000	15,381,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2018, 2
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	65,000,000
Carrying Value	14,816,000	14,869,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2018, 3
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	33,000,000
Carrying Value	26,947,000	26,927,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of December 2018, 4
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	175,000,000
Carrying Value	59,797,000	59,723,000
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	12,192,000	12,174,000
Total floating rate | Mezzanine Loan with an Initial Maturity of January 2019, 2
Debt Investments Held [Abstract]
Future Funding Obligations	4,493,000
Senior Financing	27,852,000
Carrying Value	9,457,000	8,550,000
Total floating rate | Mezzanine Loan with an Initial Maturity of March 2019
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, 2 [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	38,000,000
Carrying Value	21,951,000	21,939,000
Total floating rate | Mezzanine Loan with an Initial Maturity Date of April 2019
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	175,000,000
Carrying Value	37,307,000	37,250,000
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	24,862,000	24,830,000
Total floating rate | Mortgage/Jr Mortgage Participate Loan, Maturity Date of August 2019
Debt Investments Held [Abstract]
Future Funding Obligations	24,587,000
Senior Financing	208,094,000
Carrying Value	74,944,000	71,832,000
Total floating rate | Mezzanine Loan, Maturity of September 2019
Debt Investments Held [Abstract]
Future Funding Obligations	2,034,000
Senior Financing	189,829,000
Carrying Value	37,867,000	37,851,000
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of September 2019
Debt Investments Held [Abstract]
Future Funding Obligations	22,570,000
Senior Financing	0
Carrying Value	146,517,000	143,919,000
Total floating rate | Mezzanine Loan with an Initial Maturity of October 2019 [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	0
Senior Financing	350,000,000
Carrying Value	34,773,000	34,737,000
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity Date of December 2019 [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	1,792,000
Senior Financing	0
Carrying Value	66,026,000	0
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of January 2020
Debt Investments Held [Abstract]
Future Funding Obligations	21,303,000
Senior Financing	0
Carrying Value	48,245,000	43,845,000
Total floating rate | Mezzanine Loan with an Initial Maturity of January 2020
Debt Investments Held [Abstract]
Future Funding Obligations	1,592,000
Senior Financing	568,461,000
Carrying Value	77,927,000	75,834,000
Total floating rate | Mortgage Loan with an Initial Maturity of February 2020 [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	16,038,000
Senior Financing	0
Carrying Value	83,465,000	0
Total floating rate | Mezzanine Loan with an Initial Maturity of July 2020 [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	6,654,000
Senior Financing	35,112,000
Carrying Value	11,539,000	11,259,000
Total floating rate | Mezzanine Loan with an Initial Maturity of July 2020 [Domain]
Debt Investments Held [Abstract]
Future Funding Obligations	47,557,000
Senior Financing	326,552,000
Carrying Value	79,477,000	75,428,000
Total floating rate | Mortgage/Mezzanine Loan with an Initial Maturity of December 2020 [Member]
Debt Investments Held [Abstract]
Future Funding Obligations	40,798,000
Senior Financing	0
Carrying Value	90,789,000	88,989,000
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	0
Carrying Value	35,179,000	35,152,000
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	60,000,000
Carrying Value	15,642,000	15,635,000
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	35,074,000	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	162,553,000
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	$ 74,755,000