LONG-TERM DEBT / INTEREST EXPENSE - Senior Credit Facility: Issuance (Details) - USD ($)	Dec. 31, 2017	Mar. 04, 2016	Mar. 03, 2016
Revolving Credit Facility
Debt Instrument [Line Items]
Amount available for borrowings under credit facility	$ 225,000,000.0	$ 300,000,000.0	$ 130,000,000.0

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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