SIGNIFICANT ACCOUNTING POLICIES - Invest, VEBA, NP (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
VEBA trust
Contribution by entity to VEBA trust	$ 20,024	$ 0
Cash and cash equivalents held by VEBA trust	96,314	110,899	$ 96,314
Goodwill and indefinite-lived intangible assets:
Impairments of intangibles		0	0	$ 0
VEBA Trust \| Level 1
VEBA trust
Cash and cash equivalents held by VEBA trust	$ 19,713	$ 15,921	$ 19,713

Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3044-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6676-107765

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant