Segment Reporting - Reportable Operating Segments (Details) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of segments | segment							5
Net interest income (expense)	$ 25,223			$ 26,111			$ 78,017	$ 71,996
Property and other income	41,708			64,312			138,992	192,824
Management fee expense	(7,083)			(11,355)			(22,235)	(34,070)
Property operating expense	(15,277)			(29,756)			(54,119)	(86,076)
Transaction, investment and servicing expense	(1,627)			(1,433)			(7,668)	(3,013)
Interest expense on real estate	(12,205)			(14,281)			(37,101)	(41,786)
Depreciation and amortization	(14,770)			(25,934)			(46,766)	(82,853)
Provision for loan losses	(10,404)			(110,314)			(80,285)	(220,572)
Impairment of operating real estate	(3,451)			(272,722)			(33,512)	(282,846)
Administrative expense	(5,780)			(7,732)			(19,569)	(22,395)
Unrealized gain on mortgage loans and obligations held in securitization trusts, net	(13,162)			(1,976)			(41,589)	4,602
Realized gain on mortgage loans and obligations held in securitization trusts, net	0			2,724			0	2,772
Other gain (loss), net	9,680			(2,688)			(130,115)	(13,829)
Loss before equity in earnings of unconsolidated ventures and income taxes	(7,148)			(385,044)			(255,950)	(515,246)
Equity in earnings (loss) of unconsolidated ventures	(1,779)			(15,905)			(69,889)	17,962
Income tax benefit (expense)	15,357			(1,046)			11,544	(544)
Net income (loss)	6,430	$ (240,584)	$ (80,141)	(401,995)	$ (110,790)	$ 14,957	(314,295)	(497,828)
Accretion (amortization) of discounts and premiums, investments							1,600	1,400
Total Assets	6,511,657						6,511,657		$ 7,414,306
Equity method investments	417,464						417,464		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)	25,561			24,034			$ 78,167	63,261
Property and other income	21,183			29,094			66,652	89,229
Management fee expense	(6,445)			(9,084)			(19,446)	(27,256)
Property operating expense	(2,480)			(8,340)			(9,438)	(25,187)
Transaction, investment and servicing expense	(1,403)			(863)			(5,057)	(1,236)
Interest expense on real estate	(8,067)			(8,695)			(24,613)	(26,078)
Depreciation and amortization	(10,946)			(11,673)			(31,396)	(37,645)
Provision for loan losses	(11,229)			0			(42,642)	0
Impairment of operating real estate	0			(23,911)			0	(23,911)
Administrative expense	(4,538)			(4,171)			(13,083)	(11,977)
Unrealized gain on mortgage loans and obligations held in securitization trusts, net	(13,162)			(1,976)			(41,589)	4,602
Realized gain on mortgage loans and obligations held in securitization trusts, net				2,724				2,772
Other gain (loss), net	9,896			(2,682)			(137,178)	(12,524)
Loss before equity in earnings of unconsolidated ventures and income taxes	(1,630)			(15,543)			(179,623)	(5,950)
Equity in earnings (loss) of unconsolidated ventures	(1,652)			2,736			(72,906)	39,020
Income tax benefit (expense)	1,949			(201)			(316)	1,428
Net income (loss)	(1,333)			(13,008)			(252,845)	34,498
Total Assets	5,859,195						5,859,195		6,369,734
Core | Senior and Mezzanine Loans and Preferred Equity
Segment Reporting Information [Line Items]
Net interest income (expense)	24,677			21,295			75,309	55,077
Property and other income	(14)			209			91	450
Management fee expense	0			0			0	0
Property operating expense	0			0			(1)	0
Transaction, investment and servicing expense	(210)			(512)			(1,737)	(1,325)
Interest expense on real estate	0			0			0	0
Depreciation and amortization	0			0			0	0
Provision for loan losses	(11,229)			0			(42,642)	0
Impairment of operating real estate	0			0			0	0
Administrative expense	(66)			(312)			(802)	(614)
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				0
Other gain (loss), net	(1,457)			(15)			(49,567)	(15)
Loss before equity in earnings of unconsolidated ventures and income taxes	11,701			20,665			(19,349)	53,573
Equity in earnings (loss) of unconsolidated ventures	(1,652)			2,736			(72,906)	39,020
Income tax benefit (expense)	1,915			0			(646)	(12)
Net income (loss)	11,964			23,401			(92,901)	92,581
Total Assets	1,831,043						1,831,043		2,464,963
Core | CRE Debt Securities
Segment Reporting Information [Line Items]
Net interest income (expense)	2,338			5,102			8,929	15,856
Property and other income	0			200			73	341
Management fee expense	0			0			0	0
Property operating expense	0			0			0	0
Transaction, investment and servicing expense	(2)			(3)			(39)	(4)
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	(281)			(244)			(1,017)	(979)
Unrealized gain on mortgage loans and obligations held in securitization trusts, net	(13,750)			215			(43,154)	6,035
Realized gain on mortgage loans and obligations held in securitization trusts, net				0				48
Other gain (loss), net	1,790			(4,683)			(101,552)	(14,909)
Loss before equity in earnings of unconsolidated ventures and income taxes	(9,905)			587			(136,760)	6,388
Equity in earnings (loss) of unconsolidated ventures	0			0			0	0
Income tax benefit (expense)	0			0			0	0
Net income (loss)	(9,905)			587			(136,760)	6,388
Total Assets	1,805,418						1,805,418		2,226,448
Core | Net Leased Real Estate
Segment Reporting Information [Line Items]
Net interest income (expense)	6			0			15	2
Property and other income	21,121			28,316			66,290	88,067
Management fee expense	0			0			0	0
Property operating expense	(2,480)			(8,340)			(9,437)	(25,187)
Transaction, investment and servicing expense	(364)			(103)			(511)	(208)
Interest expense on real estate	(8,067)			(8,695)			(24,613)	(26,078)
Depreciation and amortization	(10,946)			(11,673)			(31,396)	(37,645)
Provision for loan losses	0			0			0	0
Impairment of operating real estate	0			(23,911)			0	(23,911)
Administrative expense	(28)			(78)			(231)	(178)
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				0
Other gain (loss), net	9,563			2,019			14,035	2,399
Loss before equity in earnings of unconsolidated ventures and income taxes	8,805			(22,465)			14,152	(22,739)
Equity in earnings (loss) of unconsolidated ventures	0			0			0	0
Income tax benefit (expense)	34			(201)			330	1,822
Net income (loss)	8,839			(22,666)			14,482	(20,917)
Total Assets	1,022,527						1,022,527		1,181,609
Core | Corporate
Segment Reporting Information [Line Items]
Net interest income (expense)	(1,460)			(2,363)			(6,086)	(7,674)
Property and other income	76			369			198	371
Management fee expense	(6,445)			(9,084)			(19,446)	(27,256)
Property operating expense	0			0			0	0
Transaction, investment and servicing expense	(827)			(245)			(2,770)	301
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	(4,163)			(3,537)			(11,033)	(10,206)
Unrealized gain on mortgage loans and obligations held in securitization trusts, net	588			(2,191)			1,565	(1,433)
Realized gain on mortgage loans and obligations held in securitization trusts, net				2,724				2,724
Other gain (loss), net	0			(3)			(94)	1
Loss before equity in earnings of unconsolidated ventures and income taxes	(12,231)			(14,330)			(37,666)	(43,172)
Equity in earnings (loss) of unconsolidated ventures	0			0			0	0
Income tax benefit (expense)	0			0			0	(382)
Net income (loss)	(12,231)			(14,330)			(37,666)	(43,554)
Accretion (amortization) of discounts and premiums, investments	600			2,200
Total Assets	1,200,207						1,200,207		496,714
Legacy, Non-Strategic | Legacy, Non-Strategic Portfolio
Segment Reporting Information [Line Items]
Net interest income (expense)	(338)			2,077			(150)	8,735
Property and other income	20,525			35,218			72,340	103,595
Management fee expense	(638)			(2,271)			(2,789)	(6,814)
Property operating expense	(12,797)			(21,416)			(44,681)	(60,889)
Transaction, investment and servicing expense	(224)			(570)			(2,611)	(1,777)
Interest expense on real estate	(4,138)			(5,586)			(12,488)	(15,708)
Depreciation and amortization	(3,824)			(14,261)			(15,370)	(45,208)
Provision for loan losses	825			(110,314)			(37,643)	(220,572)
Impairment of operating real estate	(3,451)			(248,811)			(33,512)	(258,935)
Administrative expense	(1,242)			(3,561)			(6,486)	(10,418)
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				0
Other gain (loss), net	(216)			(6)			7,063	(1,305)
Loss before equity in earnings of unconsolidated ventures and income taxes	(5,518)			(369,501)			(76,327)	(509,296)
Equity in earnings (loss) of unconsolidated ventures	(127)			(18,641)			3,017	(21,058)
Income tax benefit (expense)	13,408			(845)			11,860	(1,972)
Net income (loss)	7,763			$ (388,987)			(61,450)	$ (532,326)
Total Assets	$ 652,462						$ 652,462		$ 1,044,572