GOODWILL AND INTANGIBLE ASSETS - INTANGIBLE ASSETS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Intangible assets
Gross Amount	$ 258.0	$ 258.0
Accumulated Amortization	(32.8)	(21.4)
Net Amount	225.2	236.6
Aggregate intangible asset amortization	11.4	9.1	$ 4.4
Estimated amortization expenses related to intangibles
2018	11.2
2019	11.0
2020	11.0
2021	10.9
2022	10.7
Thereafter	143.1
Net Amount	197.9
Ocean Transportation
Intangible assets
Gross Amount	140.6	140.6
Accumulated Amortization	(17.8)	(11.0)
Net Amount	$ 122.8	129.6
Ocean Transportation | Customer Relationships. | Horizon
Intangible assets
Expected useful life	21 years
Finite-Lived intangible assets acquired	$ 140.6
Logistics
Intangible assets
Gross Amount	117.4	117.4
Accumulated Amortization	(15.0)	(10.4)
Net Amount	102.4	107.0
Logistics | Trade Names
Intangible assets
Indefinite-Lived intangible asset	27.3	27.3
Logistics | Span Alaska | Trade Names
Intangible assets
Indefinite-Lived intangible asset acquired	27.3
Logistics | Customer Relationships.
Intangible assets
Gross Amount	90.1	90.1
Accumulated Amortization	(15.0)	(10.4)
Net Amount	75.1	$ 79.7
Net Remaining Amount	$ 10.8
Expected useful life	13 years
Logistics | Customer Relationships. | Span Alaska
Intangible assets
Net Amount	$ 79.3
Expected useful life	20 years