SEGMENT REPORTING - Schedule of Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Statement [Abstract]
Interest income	$ 85,230	$ 65,970	$ 163,437	$ 123,482
Interest expense	(48,417)	(35,661)	(93,130)	(67,076)
Net interest income	36,813	30,309	70,307	56,406
Provision for loan losses	(300)	0	(3,300)	0
Net interest income after provision for loan losses	36,513	30,309	67,007	56,406
Operating lease income	24,258	22,187	48,818	41,816
Tenant recoveries	1,913	1,159	5,492	2,739
Sale of loans, net	6,144	25,904	11,032	24,905
Realized gain (loss) on securities	(1,243)	7,132	(2,342)	12,494
Unrealized gain (loss) on Agency interest-only securities	110	299	313	457
Realized gain on sale of real estate, net	1,628	2,232	32,637	4,563
Fee and other income	6,477	4,574	12,728	9,039
Net result from derivative transactions	7,081	(16,022)	22,040	(18,003)
Earnings (loss) from investment in unconsolidated joint ventures	13	10	65	(63)
Loss on extinguishment of debt	0	0	(69)	(54)
Total other income	46,381	47,475	130,714	77,893
Salaries and employee benefits	(13,866)	(14,489)	(30,962)	(30,531)
Operating expenses	(5,597)	(5,829)	(11,144)	(11,308)
Real estate operating expenses	(7,836)	(8,056)	(16,654)	(15,510)
Fee expense	(799)	(1,621)	(1,641)	(2,314)
Depreciation and amortization	(10,656)	(10,125)	(21,479)	(18,717)
Total costs and expenses	(38,754)	(40,120)	(81,880)	(78,380)
Income tax (expense) benefit	(573)	(6,606)	(4,476)	(5,231)
Net income (loss)	43,567	31,058	111,365	50,688	$ 125,879
Total assets	6,389,414		6,389,414		6,025,615
Investment in unconsolidated joint ventures	35,302		35,302		35,441
Investment in FHLB stock	77,915		77,915		77,915
Operating Segment
Income Statement [Abstract]
Investment in unconsolidated joint ventures	35,300		35,300		35,400
Operating Segment | Loans
Income Statement [Abstract]
Interest income	76,485	53,878	146,494	98,176
Interest expense	(15,488)	(11,123)	(29,054)	(17,376)
Net interest income	60,997	42,755	117,440	80,800
Provision for loan losses	(300)	0	(3,300)	0
Net interest income after provision for loan losses	60,697	42,755	114,140	80,800
Operating lease income	0	0	0	0
Tenant recoveries	0	0	0	0
Sale of loans, net	6,144	25,904	11,032	24,905
Realized gain (loss) on securities	0	0	0	0
Unrealized gain (loss) on Agency interest-only securities	0	0	0	0
Realized gain on sale of real estate, net	0	(159)	0	0
Fee and other income	3,866	1,730	6,929	3,351
Net result from derivative transactions	3,886	(10,508)	10,774	(12,189)
Earnings (loss) from investment in unconsolidated joint ventures	0	0	0	0
Loss on extinguishment of debt			(69)	0
Total other income	13,896	16,967	28,666	16,067
Salaries and employee benefits	0	(5,700)	0	(6,700)
Operating expenses	(86)	69	0	112
Real estate operating expenses	0	0	0	0
Fee expense	(615)	(1,271)	(1,232)	(1,806)
Depreciation and amortization	0	0	0	0
Total costs and expenses	(701)	(6,902)	(1,232)	(8,394)
Income tax (expense) benefit	0	0	0	0
Net income (loss)	73,892	52,820	141,574	88,473
Total assets	3,864,616		3,864,616		3,508,642
Operating Segment | Securities
Income Statement [Abstract]
Interest income	8,662	12,042	16,676	25,250
Interest expense	(1,085)	(1,869)	(1,941)	(3,722)
Net interest income	7,577	10,173	14,735	21,528
Provision for loan losses	0	0	0	0
Net interest income after provision for loan losses	7,577	10,173	14,735	21,528
Operating lease income	0	0	0	0
Tenant recoveries	0	0	0	0
Sale of loans, net	0	0	0	0
Realized gain (loss) on securities	(1,243)	7,132	(2,342)	12,494
Unrealized gain (loss) on Agency interest-only securities	110	299	313	457
Realized gain on sale of real estate, net	0	0	0	0
Fee and other income	72	0	72	0
Net result from derivative transactions	3,195	(5,514)	11,266	(5,814)
Earnings (loss) from investment in unconsolidated joint ventures	0	0	0	0
Loss on extinguishment of debt			0	0
Total other income	2,134	1,917	9,309	7,137
Salaries and employee benefits	0	0	0	0
Operating expenses	0	0	0	0
Real estate operating expenses	0	0	0	0
Fee expense	(96)	(68)	(206)	(162)
Depreciation and amortization	0	0	0	0
Total costs and expenses	(96)	(68)	(206)	(162)
Income tax (expense) benefit	0	0	0	0
Net income (loss)	9,615	12,022	23,838	28,503
Total assets	1,106,358		1,106,358		1,106,517
Operating Segment | Real Estate
Income Statement [Abstract]
Interest income	5	3	10	6
Interest expense	(8,732)	(6,373)	(16,586)	(12,923)
Net interest income	(8,727)	(6,370)	(16,576)	(12,917)
Provision for loan losses	0	0	0	0
Net interest income after provision for loan losses	(8,727)	(6,370)	(16,576)	(12,917)
Operating lease income	24,258	22,187	48,818	41,816
Tenant recoveries	1,913	1,159	5,492	2,739
Sale of loans, net	0	0	0	0
Realized gain (loss) on securities	0	0	0	0
Unrealized gain (loss) on Agency interest-only securities	0	0	0	0
Realized gain on sale of real estate, net	1,628	2,391	32,637	4,563
Fee and other income	1,634	2,011	3,416	3,984
Net result from derivative transactions	0	0	0	0
Earnings (loss) from investment in unconsolidated joint ventures	13	10	65	(63)
Loss on extinguishment of debt			0	0
Total other income	29,446	27,758	90,428	53,039
Salaries and employee benefits	0	0	0	0
Operating expenses	0	0	0	0
Real estate operating expenses	(7,836)	(8,056)	(16,654)	(15,510)
Fee expense	(88)	(282)	(203)	(346)
Depreciation and amortization	(10,637)	(10,102)	(21,441)	(18,670)
Total costs and expenses	(18,561)	(18,440)	(38,298)	(34,526)
Income tax (expense) benefit	0	0	0	0
Net income (loss)	2,158	2,948	35,554	5,596
Total assets	1,095,545		1,095,545		1,067,482
Corporate/Other
Income Statement [Abstract]
Interest income	78	47	257	50
Interest expense	(23,112)	(16,296)	(45,549)	(33,055)
Net interest income	(23,034)	(16,249)	(45,292)	(33,005)
Provision for loan losses	0	0	0	0
Net interest income after provision for loan losses	(23,034)	(16,249)	(45,292)	(33,005)
Operating lease income	0	0	0	0
Tenant recoveries	0	0	0	0
Sale of loans, net	0	0	0	0
Realized gain (loss) on securities	0	0	0	0
Unrealized gain (loss) on Agency interest-only securities	0	0	0	0
Realized gain on sale of real estate, net	0	0	0	0
Fee and other income	905	833	2,311	1,704
Net result from derivative transactions	0	0	0	0
Earnings (loss) from investment in unconsolidated joint ventures	0	0	0	0
Loss on extinguishment of debt			0	(54)
Total other income	905	833	2,311	1,650
Salaries and employee benefits	(13,866)	(8,789)	(30,962)	(23,831)
Operating expenses	(5,511)	(5,898)	(11,144)	(11,420)
Real estate operating expenses	0	0
Fee expense	0	0	0	0
Depreciation and amortization	(19)	(23)	(38)	(47)
Total costs and expenses	(19,396)	(14,710)	(42,144)	(35,298)
Income tax (expense) benefit	(573)	(6,606)	(4,476)	(5,231)
Net income (loss)	(42,098)	$ (36,732)	(89,601)	$ (71,884)
Total assets	322,895		322,895		342,974
Investment in FHLB stock	77,900		77,900		77,900
Deferred tax liabilities	(6,900)		(6,900)		(5,700)
Corporate/Other | Senior Unsecured Notes
Income Statement [Abstract]
Senior Notes	$ 1,200,000		$ 1,200,000		$ 1,200,000