BORROWINGS - Maturities of Repurchase Agreements (Details)	Dec. 31, 2017 USD ($)
Repurchase Agreements [Line Items]
Repurchase agreements	$ 1,234,522,000
Maturity Less Than or Equal to 30 Days
Repurchase Agreements [Line Items]
Repurchase agreements	1,175,407,000
Maturity 31 to 60 Days
Repurchase Agreements [Line Items]
Repurchase agreements	56,560,000
Maturity 61 to 90 Days
Repurchase Agreements [Line Items]
Repurchase agreements	$ 2,555,000

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 10
-Section 55
-Paragraph 51
-URI http://asc.fasb.org/extlink&oid=109221364&loc=d3e101761-111710

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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

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