DEBT OBLIGATIONS, NET - Schedule of Company's Debt Obligations (Details)				3 Months Ended		12 Months Ended
	Mar. 21, 2017 USD ($) Extension	Feb. 22, 2017	Aug. 03, 2016 USD ($) Extension	Mar. 31, 2017 USD ($) Extension	Mar. 31, 2016	Dec. 31, 2016 USD ($) Extension	Apr. 03, 2017 USD ($)	Jul. 01, 2016 USD ($)	Jun. 28, 2016 USD ($)	Apr. 19, 2016 Extension
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding				$ 1,039,355,000		$ 1,107,185,000
Debt obligations, net				4,377,686,000		3,942,138,000
Carrying Amount of Collateral				0		0
Unamortized debt issuance costs				(9,156,000)
Committed Loan Repurchase Facilities
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				1,650,000,000		1,650,000,000
Debt obligations outstanding				684,994,000		567,163,000
Committed but Unfunded				965,006,000		1,082,837,000
Carrying Amount of Collateral				1,110,581,000		961,791,000
Fair Value of Collateral				1,134,516,000		965,888,000
Committed Loan Repurchase Facilities | 10/30/2018
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				600,000,000		600,000,000
Debt obligations outstanding				311,312,000		183,604,000
Committed but Unfunded				288,688,000		416,396,000
Carrying Amount of Collateral				453,843,000		292,628,000
Fair Value of Collateral				$ 461,210,000		$ 293,618,000
Number of extension maturity periods | Extension				3		3
Length of extension options				12 months	1 year	12 months
Committed Loan Repurchase Facilities | 10/30/2018 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				2.66%		2.45%
Committed Loan Repurchase Facilities | 10/30/2018 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				3.41%		3.27%
Committed Loan Repurchase Facilities | 5/24/2017
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				$ 450,000,000		$ 450,000,000
Debt obligations outstanding				137,324,000		184,158,000
Committed but Unfunded				312,676,000		265,842,000
Carrying Amount of Collateral				241,974,000		286,848,000
Fair Value of Collateral				$ 244,538,000		$ 288,267,000
Number of extension maturity periods | Extension				3		3				2
Length of extension options				12 months		12 months
Committed Loan Repurchase Facilities | 5/24/2017 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				3.08%		2.95%
Committed Loan Repurchase Facilities | 5/24/2017 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				3.90%		3.70%
Committed Loan Repurchase Facilities | 4/9/2017
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement						$ 400,000,000
Debt obligations outstanding						100,979,000
Committed but Unfunded						299,021,000
Carrying Amount of Collateral						235,878,000
Fair Value of Collateral						$ 236,696,000
Number of extension maturity periods | Extension						2
Length of extension options						364 days
Committed Loan Repurchase Facilities | 4/9/2017 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)						2.95%
Committed Loan Repurchase Facilities | 4/9/2017 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)						3.99%
Committed Loan Repurchase Facilities | 4/9/2018
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement	$ 300,000,000.0			$ 300,000,000
Debt obligations outstanding				151,257,000
Committed but Unfunded				148,743,000
Carrying Amount of Collateral				299,156,000
Fair Value of Collateral				$ 306,986,000
Number of extension maturity periods | Extension				2
Length of extension options		1 year
Number of additional extension maturity periods | Extension	1			1
Length of additional extension maturity periods	364 days			364 days
Committed Loan Repurchase Facilities | 4/9/2018 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				3.08%
Committed Loan Repurchase Facilities | 4/9/2018 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				4.16%
Committed Loan Repurchase Facilities | 6/28/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				$ 100,000,000		$ 100,000,000			$ 100,000,000.0
Debt obligations outstanding				34,369,000		27,132,000
Committed but Unfunded				65,631,000		72,868,000
Carrying Amount of Collateral				46,186,000		36,166,000
Fair Value of Collateral				$ 46,491,000		$ 36,410,000
Committed Loan Repurchase Facilities | 6/28/2019 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				3.16%		2.90%
Committed Loan Repurchase Facilities | 6/28/2019 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				3.41%		3.13%
Committed Loan Repurchase Facilities | 8/2/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement			$ 100,000,000.0			$ 100,000,000
Debt obligations outstanding						71,290,000
Committed but Unfunded						28,710,000
Carrying Amount of Collateral						110,271,000
Fair Value of Collateral						$ 110,897,000
Number of extension maturity periods | Extension			1			1
Length of extension options			12 months			12 months
Number of additional extension maturity periods | Extension			2			2
Length of additional extension maturity periods			6 months			6 months
Committed Loan Repurchase Facilities | 8/2/2019 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)						2.93%
Committed Loan Repurchase Facilities | 8/2/2019 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)						3.68%
Committed Loan Repurchase Facilities | 2/29/2020
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				$ 200,000,000
Debt obligations outstanding				50,732,000
Committed but Unfunded				149,268,000
Carrying Amount of Collateral				69,422,000
Fair Value of Collateral				$ 75,291,000
Number of extension maturity periods | Extension				1
Length of extension options				12 months
Number of additional extension maturity periods | Extension				2
Length of additional extension maturity periods				6 months
Committed Loan Repurchase Facilities | 2/29/2020 | Subsidiaries
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of Collateral				$ 5,100,000
Committed Loan Repurchase Facilities | 2/29/2020 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				3.13%
Committed Loan Repurchase Facilities | 2/29/2020 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				3.88%
Committed Securities Repurchase Facility
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				$ 400,000,000.0
Committed Securities Repurchase Facility | 7/1/2018
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				400,000,000		$ 400,000,000		$ 400,000,000.0
Debt obligations outstanding				115,000,000		228,317,000
Committed but Unfunded				285,000,000		171,683,000
Carrying Amount of Collateral				213,238,000		272,402,000
Fair Value of Collateral				$ 213,238,000		$ 272,402,000
Committed Securities Repurchase Facility | 7/1/2018 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				1.10%		1.00%
Committed Securities Repurchase Facility | 7/1/2018 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				2.12%		2.59%
Uncommitted Securities Repurchase Facilities | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding				$ 239,361,000		$ 311,705,000
Carrying Amount of Collateral				279,764,000		368,638,000
Fair Value of Collateral				$ 279,764,000		$ 368,638,000
Uncommitted Securities Repurchase Facilities | Various Dates | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				1.10%		1.00%
Uncommitted Securities Repurchase Facilities | Various Dates | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				2.57%		2.41%
Total Repurchase Facilities
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				$ 2,050,000,000		$ 2,050,000,000
Debt obligations outstanding				1,039,355,000		1,107,185,000
Committed but Unfunded				1,250,006,000		1,254,520,000
Carrying Amount of Collateral				1,603,583,000		1,602,831,000
Fair Value of Collateral				1,627,518,000		1,606,928,000
Revolving Credit Facility | 2/11/2017
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement						143,000,000
Debt obligations outstanding						25,000,000
Committed but Unfunded						$ 118,000,000
Interest Rate(s) (as a percent)						3.16%
Revolving Credit Facility | 2/11/2018
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				168,520,000
Debt obligations outstanding				168,000,000
Committed but Unfunded				$ 520,000
Interest Rate(s) (as a percent)				6.50%
Mortgage Loan Financing | 2/11/2017
Assets Sold under Agreements to Repurchase [Line Items]
Number of extension maturity periods | Extension						2
Length of extension options						12 months
Mortgage Loan Financing | 2/11/2018
Assets Sold under Agreements to Repurchase [Line Items]
Number of extension maturity periods | Extension				3
Length of extension options				12 months
Mortgage Loan Financing | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				$ 589,217,000		$ 590,106,000
Debt obligations outstanding				589,217,000		590,106,000
Committed but Unfunded				0		0
Carrying Amount of Collateral				749,677,000		757,468,000
Fair Value of Collateral				$ 875,725,000		$ 875,160,000
Mortgage Loan Financing | Various Dates | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				4.25%		4.25%
Mortgage Loan Financing | Various Dates | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				6.75%		6.75%
Mortgage Loan Receivable Financing
Assets Sold under Agreements to Repurchase [Line Items]
Mortgage loans financing				$ 57,038,000
Mortgage Loan Receivable Financing | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				57,038,000
Committed but Unfunded				0
Borrowings from the FHLB | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				2,000,000,000		$ 1,998,931,000
Debt obligations outstanding				1,475,500,000		1,660,000,000
Committed but Unfunded				524,500,000		338,931,000
Carrying Amount of Collateral				1,921,263,000		2,162,779,000
Fair Value of Collateral				$ 1,930,137,000		$ 2,167,017,000
Borrowings from the FHLB | Various Dates | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				0.57%		0.43%
Borrowings from the FHLB | Various Dates | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				2.74%		2.74%
Senior Unsecured Notes | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Debt issued				$ 1,057,732,000		$ 563,872,000
Senior unsecured notes				1,048,576,000		559,847,000
Committed but Unfunded				0		0
Unamortized debt issuance costs				(9,200,000)		$ (4,000,000.0)
Debt instrument, repurchase price amount				$ 291,500,000
Senior Unsecured Notes | Various Dates | Subsequent Event
Assets Sold under Agreements to Repurchase [Line Items]
Debt instrument, repurchase price amount							$ 291,500,000
Senior Unsecured Notes | Various Dates | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Stated interest rate on debt instrument (as a percent)				5.25%		5.875%
Senior Unsecured Notes | Various Dates | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Stated interest rate on debt instrument (as a percent)				7.375%		7.375%
Total Debt Obligations
Assets Sold under Agreements to Repurchase [Line Items]
Debt issued				$ 5,922,507,000		$ 5,345,909,000
Debt obligations, net				4,377,686,000		3,942,138,000
Committed but Unfunded				1,775,026,000		1,711,451,000
Carrying Amount of Collateral				4,274,523,000		4,523,078,000
Fair Value of Collateral				$ 4,433,380,000		$ 4,649,105,000