REVENUE, CONTRACT ASSETS AND LIABILITIES AND OTHER CONTRACT MATTERS - Narrative (Details) $ in Thousands	3 Months Ended
Mar. 31, 2019 USD ($) vessel	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Long-term Contracts or Programs Disclosure [Line Items]
Bad debt expense	$ 53	$ 8
Allowance for bad debt	$ 400		$ 400
Number of harbor tug vessels \| vessel	8
Shipyard
Long-term Contracts or Programs Disclosure [Line Items]
Reduction to estimated contract price	$ 1,300

Amount of increase (decrease) in revenue recognized for cumulative catch-up adjustment from change in estimate of transaction price which increases (decreases) right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time. Includes, but is not limited to, change in assessment of whether estimate of variable consideration is constrained.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130551-203045

Name:	us-gaap_ContractWithCustomerAssetCumulativeCatchUpAdjustmentToRevenueChangeInEstimateOfTransactionPrice
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration