BORROWINGS - Maturities of Repurchase Agreements (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Repurchase Agreements [Line Items]
Secured borrowings	$ 1,234,522,000	$ 804,811,000
Maturity Less Than or Equal to 30 Days
Repurchase Agreements [Line Items]
Secured borrowings	1,175,407,000	737,823,000
Maturity 31 to 60 Days
Repurchase Agreements [Line Items]
Secured borrowings	56,560,000	19,897,000
Maturity 61 to 90 Days
Repurchase Agreements [Line Items]
Secured borrowings	$ 2,555,000	$ 47,091,000

Carrying value, as of the balance sheet date, of securities sold under agreements to repurchase where the transferor maintains effective control over the assets, accounting for them as secured debt.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(1)(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 10
-Section 55
-Paragraph 51
-URI http://asc.fasb.org/extlink&oid=77903054&loc=d3e101761-111710

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