DEBT OBLIGATIONS - Narrative (Details) $ in Thousands	Mar. 31, 2018 USD ($) agreement
Debt Instrument [Line Items]
Face amount of debt	$ 14,685,727
Repurchase Agreements \| Total Repurchase Agreements
Debt Instrument [Line Items]
Face amount of debt	$ 7,636,801
Repurchase Agreements \| Stockholders' Equity, Total \| Counterpary Concentration Risk
Debt Instrument [Line Items]
Number of repurchase agreements, outstanding \| agreement	0

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant