SEGMENT REPORTING - Schedule of Segments (Details) - USD ($) $ in Thousands		3 Months Ended										12 Months Ended
		Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Income Statement [Abstract]
Interest income		$ 90,994		$ 90,386	$ 85,230	$ 78,206	$ 73,352		$ 66,833	$ 65,970	$ 57,512	$ 344,816		$ 263,667		$ 236,372
Interest expense												(194,291)		(146,118)		(120,827)
Net interest income												150,525		117,549		115,545
Provision for loan losses												(13,900)		0		(300)
Net interest income after provision for loan losses		41,009		28,610	36,513	30,493	31,795		29,348	30,309	26,097	136,625		117,549		115,245
Operating lease income												96,506		89,492		77,277
Tenant recoveries												9,671		7,179		5,958
Sale of loans, net												16,511		54,046		26,009
Gain on securities																7,724
Realized gain (loss) on securities												(5,808)		17,209		7,724
Unrealized gain (loss) on equity securities												(1,605)		0		0
Unrealized gain (loss) on Agency interest-only securities												555		1,405		(56)
Realized gain on sale of real estate, net												95,881		11,423		20,636
Fee and other income												26,285		18,341		21,365
Net result from derivative transactions												15,926		(12,641)		(1,409)
Earnings (loss) from investment in unconsolidated joint ventures												790		89		426
Gain (loss) on extinguishment/defeasance of debt												(4,392)		(73)		5,382
Total other income												250,320		186,470		163,312
Salaries and employee benefits												(60,117)		(70,463)		(64,270)
Operating expenses												(21,696)		(21,421)		(20,552)
Real estate operating expenses												(29,799)		(33,216)		(30,545)
Fee expense												(5,055)		(4,996)		(3,703)
Depreciation and amortization												(41,959)		(40,332)		(39,447)
Total costs and expenses		(36,610)		(40,136)	(38,753)	(43,127)	(52,804)		(39,244)	(40,120)	(38,260)	(158,626)		(170,428)		(158,517)
Income tax (expense) benefit		(964)		(1,204)	(573)	(3,902)	(3,057)		576	(6,606)	1,375	(6,643)		(7,712)		(6,320)
Net income (loss)		26,846		$ 83,464	$ 43,568	$ 67,798	45,370		$ 29,821	$ 31,058	$ 19,630	221,676		125,879		113,720
Total assets		6,272,872	[1]				6,025,615	[1]				6,272,872	[1]	6,025,615	[1]	5,578,337
Investment in unconsolidated joint ventures	[1]	40,354					35,441					40,354		35,441
Investment in FHLB stock	[1]	57,915					77,915					57,915		77,915
Deferred tax liabilities		0					(7,134)					0		(7,134)
Professional Fees												11,500		11,200		11,300
Operating Segment
Income Statement [Abstract]
Investment in unconsolidated joint ventures		40,400					35,400					40,400		35,400
Operating Segment | Loans
Income Statement [Abstract]
Interest income												310,149		219,892		161,315
Interest expense												(62,474)		(39,530)		(25,531)
Net interest income												247,675		180,362		135,784
Provision for loan losses												(13,900)		0		(300)
Net interest income after provision for loan losses												233,775		180,362		135,484
Operating lease income												0		0		0
Tenant recoveries												0		0		0
Sale of loans, net												16,511		54,046		26,009
Gain on securities																0
Realized gain (loss) on securities												0		0
Unrealized gain (loss) on equity securities												0
Unrealized gain (loss) on Agency interest-only securities												0		0		0
Realized gain on sale of real estate, net												0		0		0
Fee and other income												16,490		6,859		7,547
Net result from derivative transactions												10,467		(8,425)		8,371
Earnings (loss) from investment in unconsolidated joint ventures												0		0		0
Gain (loss) on extinguishment/defeasance of debt												(69)		(19)		0
Total other income												43,399		52,461		41,927
Salaries and employee benefits												0		0		(11,000)
Operating expenses												0		302		0
Real estate operating expenses												0		0		0
Fee expense												(4,040)		(3,649)		(2,343)
Depreciation and amortization												0		0		0
Total costs and expenses												(4,040)		(3,347)		(13,343)
Income tax (expense) benefit												0		0		0
Net income (loss)												273,134		229,476		164,068
Total assets		3,482,929					3,508,642					3,482,929		3,508,642		2,353,977
Operating Segment | Securities
Income Statement [Abstract]
Interest income												34,217		43,542		74,987
Interest expense												(4,617)		(5,800)		(9,740)
Net interest income												29,600		37,742		65,247
Provision for loan losses												0		0		0
Net interest income after provision for loan losses												29,600		37,742		65,247
Operating lease income												0		0		0
Tenant recoveries												0		0		0
Sale of loans, net												0		0		0
Gain on securities																7,724
Realized gain (loss) on securities												(5,808)		17,209
Unrealized gain (loss) on equity securities												(1,605)
Unrealized gain (loss) on Agency interest-only securities												555		1,405		(56)
Realized gain on sale of real estate, net												0		0		0
Fee and other income												0		0		0
Net result from derivative transactions												5,459		(4,216)		(9,780)
Earnings (loss) from investment in unconsolidated joint ventures												0		0		0
Gain (loss) on extinguishment/defeasance of debt												0		0		0
Total other income												(1,399)		14,398		(2,112)
Salaries and employee benefits												0		0		0
Operating expenses												0		0		0
Real estate operating expenses												0		0		0
Fee expense												(398)		(280)		(166)
Depreciation and amortization												0		0		0
Total costs and expenses												(398)		(280)		(166)
Income tax (expense) benefit												0		0		0
Net income (loss)												27,803		51,860		62,969
Total assets		1,410,126					1,106,517					1,410,126		1,106,517		2,100,947
Operating Segment | Real Estate
Income Statement [Abstract]
Interest income												24		15		10
Interest expense												(34,739)		(28,679)		(25,333)
Net interest income												(34,715)		(28,664)		(25,323)
Provision for loan losses												0		0		0
Net interest income after provision for loan losses												(34,715)		(28,664)		(25,323)
Operating lease income												96,506		89,492		77,277
Tenant recoveries												9,671		7,179		5,958
Sale of loans, net												0		0		0
Gain on securities																0
Realized gain (loss) on securities												0		0
Unrealized gain (loss) on equity securities												0
Unrealized gain (loss) on Agency interest-only securities												0		0		0
Realized gain on sale of real estate, net												95,881		11,423		20,636
Fee and other income												3,416		7,865		7,253
Net result from derivative transactions												0		0		0
Earnings (loss) from investment in unconsolidated joint ventures												790		89		(466)
Gain (loss) on extinguishment/defeasance of debt												(4,323)		0		0
Total other income												201,941		116,048		110,658
Salaries and employee benefits												0		0		0
Operating expenses												0		0		0
Real estate operating expenses												(29,799)		(33,216)		(30,545)
Fee expense												(617)		(1,067)		(618)
Depreciation and amortization												(41,884)		(40,239)		(39,354)
Total costs and expenses												(72,300)		(74,522)		(70,517)
Income tax (expense) benefit												0		0		0
Net income (loss)												94,926		12,862		14,818
Total assets		1,038,376					1,067,482					1,038,376		1,067,482		856,363
Corporate/Other
Income Statement [Abstract]
Interest income												426		218		60
Interest expense												(92,461)		(72,109)		(60,223)
Net interest income												(92,035)		(71,891)		(60,163)
Provision for loan losses												0		0		0
Net interest income after provision for loan losses												(92,035)		(71,891)		(60,163)
Operating lease income												0		0		0
Tenant recoveries												0		0		0
Sale of loans, net												0		0		0
Gain on securities																0
Realized gain (loss) on securities												0		0
Unrealized gain (loss) on equity securities												0
Unrealized gain (loss) on Agency interest-only securities												0		0		0
Realized gain on sale of real estate, net												0		0		0
Fee and other income												6,379		3,617		6,565
Net result from derivative transactions												0		0		0
Earnings (loss) from investment in unconsolidated joint ventures												0		0		892
Gain (loss) on extinguishment/defeasance of debt												0		(54)		5,382
Total other income												6,379		3,563		12,839
Salaries and employee benefits												(60,117)		(70,463)		(53,270)
Operating expenses												(21,696)		(21,723)		(20,552)
Real estate operating expenses														0		0
Fee expense												0		0		(576)
Depreciation and amortization												(75)		(93)		(93)
Total costs and expenses												(81,888)		(92,279)		(74,491)
Income tax (expense) benefit												(6,643)		(7,712)		(6,320)
Net income (loss)												(174,187)		(168,319)		(128,135)
Total assets		341,441					342,974					341,441		342,974		$ 267,050
Investment in FHLB stock		57,900					77,900					57,900		77,900
Deferred tax liabilities		2,300					(5,700)					2,300		(5,700)
Corporate/Other | Senior Unsecured Notes
Income Statement [Abstract]
Senior Notes		$ 1,200,000					$ 1,200,000					$ 1,200,000		$ 1,200,000

[1] Includes amounts relating to consolidated variable interest entities. See Note 3.