LOAN SALES AND SECURITIZATIONS - Automobile Loans, MSRs Amortizations Method Key Assumptions (Details)	3 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Amortization method \| Automobile
Servicing Asset at Amortized Cost [Line Items]
Spread over forward interest rate swap rates, actual	0.00%	0.00%

Discount rate used to estimate the fair value of servicing assets and servicing liabilities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122596-111746

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration