SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - AOCI Components (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Details about AOCI Components
Investment income	$ (3,673)	$ (595)
Reclassification Out Of Accumulated Other Comprehensive Income (Loss)
Details about AOCI Components
Investment income	24	(2)
Unrealized Gain (Loss) on Securities \| Reclassification Out Of Accumulated Other Comprehensive Income (Loss)
Details about AOCI Components
Investment income	$ 24	$ (2)

Amount before accretion (amortization) of purchase discount (premium) of interest income on nonoperating securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7(b))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration