Segment Reporting - Reportable Operating Segments (Details) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of segments | segment					5
Net interest income (expense)	$ 24,938		$ 23,927		$ 52,794	$ 45,885
Property and other income	35,362		65,201		97,284	128,512
Management fee expense	(7,206)		(11,357)		(15,152)	(22,715)
Property operating expense	(16,311)		(28,140)		(38,842)	(56,320)
Transaction, investment and servicing expense	(2,907)		(1,051)		(6,041)	(1,580)
Interest expense on real estate	(11,818)		(13,898)		(24,896)	(27,505)
Depreciation and amortization	(14,020)		(29,257)		(31,996)	(56,919)
Provision for loan losses	51		(110,258)		(69,881)	(110,258)
Impairment of operating real estate	(25,935)		(10,124)		(30,061)	(10,124)
Administrative expense	(6,751)		(8,010)		(13,789)	(14,663)
Unrealized gain on mortgage loans and obligations held in securitization trusts, net	(8,975)		5,549		(28,427)	6,578
Realized gain on mortgage loans and obligations held in securitization trusts, net	0		0		0	48
Other gain (loss), net	(119,633)		(6,062)		(139,795)	(11,141)
Loss before equity in earnings of unconsolidated ventures and income taxes	(153,205)		(123,480)		(248,802)	(130,202)
Equity in earnings (loss) of unconsolidated ventures	(85,277)		12,557		(68,110)	33,867
Income tax benefit (expense)	(2,102)		133		(3,813)	502
Net income (loss)	(240,584)	$ (80,141)	(110,790)	$ 14,957	(320,725)	(95,833)
Accretion (amortization) of discounts and premiums, investments					1,000	800
Total Assets	6,850,267				6,850,267		$ 7,414,306
Equity method investments	410,214				410,214		585,022
PE Investments	7,093				$ 7,093		10,283
Core
Segment Reporting Information [Line Items]
Number of segments | segment					4
Net interest income (expense)	25,449		20,891		$ 52,605	39,225
Property and other income	14,833		30,070		45,466	60,134
Management fee expense	(6,485)		(9,086)		(13,002)	(18,172)
Property operating expense	(3,275)		(7,901)		(6,958)	(16,847)
Transaction, investment and servicing expense	(1,440)		(317)		(3,654)	(372)
Interest expense on real estate	(8,085)		(8,814)		(16,546)	(17,384)
Depreciation and amortization	(9,297)		(12,888)		(20,449)	(25,972)
Provision for loan losses	86		0		(31,413)	0
Impairment of operating real estate	0		0		0	0
Administrative expense	(4,412)		(4,168)		(8,543)	(7,808)
Unrealized gain on mortgage loans and obligations held in securitization trusts, net	(8,975)		5,549		(28,427)	6,578
Realized gain on mortgage loans and obligations held in securitization trusts, net						48
Other gain (loss), net	(126,562)		(6,016)		(147,074)	(9,842)
Loss before equity in earnings of unconsolidated ventures and income taxes	(128,163)		7,320		(177,995)	9,588
Equity in earnings (loss) of unconsolidated ventures	(85,328)		17,917		(71,255)	36,285
Income tax benefit (expense)	(2,101)		(359)		(2,264)	1,629
Net income (loss)	(215,592)		24,878		(251,514)	47,502
Total Assets	6,192,208				6,192,208		6,369,734
Core | Senior and Mezzanine Loans and Preferred Equity
Segment Reporting Information [Line Items]
Net interest income (expense)	27,148		17,902		50,631	33,781
Property and other income	81		148		105	241
Management fee expense	0		0		0	0
Property operating expense	0		0		(1)	0
Transaction, investment and servicing expense	(1,130)		(537)		(1,528)	(813)
Interest expense on real estate	0		0		0	0
Depreciation and amortization	0		0		0	0
Provision for loan losses	86		0		(31,413)	0
Impairment of operating real estate	0		0		0	0
Administrative expense	(372)		(13)		(735)	(303)
Unrealized gain on mortgage loans and obligations held in securitization trusts, net	0		0		0	0
Realized gain on mortgage loans and obligations held in securitization trusts, net						0
Other gain (loss), net	(48,110)		0		(48,110)	0
Loss before equity in earnings of unconsolidated ventures and income taxes	(22,297)		17,500		(31,051)	32,906
Equity in earnings (loss) of unconsolidated ventures	(85,328)		17,917		(71,255)	36,285
Income tax benefit (expense)	(2,199)		0		(2,560)	(12)
Net income (loss)	(109,824)		35,417		(104,866)	69,179
Total Assets	2,150,288				2,150,288		2,464,963
Core | CRE Debt Securities
Segment Reporting Information [Line Items]
Net interest income (expense)	1,051		5,442		6,593	10,755
Property and other income	0		74		72	141
Management fee expense	0		0		0	0
Property operating expense	0		0		0	0
Transaction, investment and servicing expense	(37)		0		(37)	0
Interest expense on real estate	0		0		0	0
Depreciation and amortization	0		0		0	0
Provision for loan losses	0		0		0	0
Impairment of operating real estate	0		0		0	0
Administrative expense	(200)		(348)		(735)	(735)
Unrealized gain on mortgage loans and obligations held in securitization trusts, net	(9,498)		5,154		(29,404)	5,820
Realized gain on mortgage loans and obligations held in securitization trusts, net						48
Other gain (loss), net	(87,006)		(6,156)		(103,342)	(10,227)
Loss before equity in earnings of unconsolidated ventures and income taxes	(95,690)		4,166		(126,853)	5,802
Equity in earnings (loss) of unconsolidated ventures	0		0		0	0
Income tax benefit (expense)	0		0		0	0
Net income (loss)	(95,690)		4,166		(126,853)	5,802
Total Assets	1,843,041				1,843,041		2,226,448
Core | Net Leased Real Estate
Segment Reporting Information [Line Items]
Net interest income (expense)	2		0		9	0
Property and other income	14,636		29,847		45,168	59,752
Management fee expense	1		0		0	0
Property operating expense	(3,275)		(7,901)		(6,957)	(16,847)
Transaction, investment and servicing expense	(4)		(60)		(147)	(105)
Interest expense on real estate	(8,085)		(8,814)		(16,546)	(17,384)
Depreciation and amortization	(9,297)		(12,888)		(20,449)	(25,972)
Provision for loan losses	0		0		0	0
Impairment of operating real estate	0		0		0	0
Administrative expense	(121)		(43)		(203)	(100)
Unrealized gain on mortgage loans and obligations held in securitization trusts, net	0		0		0	0
Realized gain on mortgage loans and obligations held in securitization trusts, net						0
Other gain (loss), net	8,556		144		4,472	380
Loss before equity in earnings of unconsolidated ventures and income taxes	2,413		285		5,347	(276)
Equity in earnings (loss) of unconsolidated ventures	0		0		0	0
Income tax benefit (expense)	98		(359)		296	2,023
Net income (loss)	2,511		(74)		5,643	1,747
Total Assets	1,131,680				1,131,680		1,181,609
Core | Corporate
Segment Reporting Information [Line Items]
Net interest income (expense)	(2,752)		(2,453)		(4,628)	(5,311)
Property and other income	116		1		121	0
Management fee expense	(6,486)		(9,086)		(13,002)	(18,172)
Property operating expense	0		0		0	0
Transaction, investment and servicing expense	(269)		280		(1,942)	546
Interest expense on real estate	0		0		0	0
Depreciation and amortization	0		0		0	0
Provision for loan losses	0		0		0	0
Impairment of operating real estate	0		0		0	0
Administrative expense	(3,719)		(3,764)		(6,870)	(6,670)
Unrealized gain on mortgage loans and obligations held in securitization trusts, net	523		395		977	758
Realized gain on mortgage loans and obligations held in securitization trusts, net						0
Other gain (loss), net	(2)		(4)		(94)	5
Loss before equity in earnings of unconsolidated ventures and income taxes	(12,589)		(14,631)		(25,438)	(28,844)
Equity in earnings (loss) of unconsolidated ventures	0		0		0	0
Income tax benefit (expense)	0		0		0	(382)
Net income (loss)	(12,589)		(14,631)		(25,438)	(29,226)
Accretion (amortization) of discounts and premiums, investments	500		400
Total Assets	1,067,199				1,067,199		496,714
Legacy, Non-Strategic | Legacy, Non-Strategic Portfolio
Segment Reporting Information [Line Items]
Net interest income (expense)	(511)		3,036		189	6,660
Property and other income	20,529		35,131		51,818	68,378
Management fee expense	(721)		(2,271)		(2,150)	(4,543)
Property operating expense	(13,036)		(20,239)		(31,884)	(39,473)
Transaction, investment and servicing expense	(1,467)		(734)		(2,387)	(1,208)
Interest expense on real estate	(3,733)		(5,084)		(8,350)	(10,121)
Depreciation and amortization	(4,723)		(16,369)		(11,547)	(30,947)
Provision for loan losses	(35)		(110,258)		(38,468)	(110,258)
Impairment of operating real estate	(25,935)		(10,124)		(30,061)	(10,124)
Administrative expense	(2,339)		(3,842)		(5,246)	(6,855)
Unrealized gain on mortgage loans and obligations held in securitization trusts, net	0		0		0	0
Realized gain on mortgage loans and obligations held in securitization trusts, net						0
Other gain (loss), net	6,929		(46)		7,279	(1,299)
Loss before equity in earnings of unconsolidated ventures and income taxes	(25,042)		(130,800)		(70,807)	(139,790)
Equity in earnings (loss) of unconsolidated ventures	51		(5,360)		3,145	(2,418)
Income tax benefit (expense)	(1)		492		(1,549)	(1,127)
Net income (loss)	(24,992)		$ (135,668)		(69,211)	$ (143,335)
Total Assets	$ 658,059				$ 658,059		$ 1,044,572