DEBT OBLIGATIONS, NET - Liability for Transfers Not Considered Sales (Non-Recourse) (Details) $ in Thousands			3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) loan	Jun. 29, 2017 USD ($) loan	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Mortgage loans on real estate, transfered to securitization		$ 625,700
Outstanding Face Amount	$ 3,989,046		$ 3,989,046		$ 3,989,046		$ 2,371,827
Risk retention requirement, amount		$ 12,900
Risk retention requirement, percentage		2.00%
Sold, horizontal interest percentage		3.00%
Controlling classes, percentage		98.00%
Purchase, not recognized for accounting purposes		$ 62,700
Interest income			72,763	$ 60,284	196,410	$ 175,650
Liability for transfers not considered sales	631,480		631,480		631,480		0
Proceeds from sales of real estate securities					983,386	308,429
Debt obligations	$ 4,828,027		4,828,027		4,828,027
Interest expense			42,607	$ 30,685	109,625	$ 88,622
First mortgage loan, held for investment
Debt Instrument [Line Items]
Number of loans | loan	34	34
Outstanding Face Amount	$ 2,703,936	$ 549,000	2,703,936		2,703,936		1,843,006
Carrying Value	546,700	$ 547,700	546,700		546,700
Intercompany Loans Held-For-Investment
Debt Instrument [Line Items]
Number of loans | loan		23
Outstanding Face Amount		$ 76,700
Mortgage loans transferred but not considered sold
Debt Instrument [Line Items]
Outstanding Face Amount	600,222		600,222		600,222		$ 0
Carrying Value	$ 598,525		598,525		598,525
Interest income			$ 7,200		$ 7,300
Liability for transfers not considered sales		580,000
Proceeds from sales of real estate securities		655,600
Purchase, not reflected in consolidated financial statement		$ 75,600
Interest rate, effective percentage	4.30%		4.30%		4.30%
Weighted average term	8 years 9 months 3 days
Interest expense			$ 6,200		$ 6,300
Non-Controlling loan interest
Debt Instrument [Line Items]
Number of loans | loan		1
Outstanding Face Amount		$ 52,300
Carrying Value		51,800
Liability for transfers not considered sales		$ 52,100
Interest rate, effective percentage		5.29%
Liability for transfers not considered sales
Debt Instrument [Line Items]
Debt obligations	$ 631,480		$ 631,480		$ 631,480