GOODWILL AND INTANGIBLE ASSETS - INTANGIBLE ASSETS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Intangible assets
Gross Amount	$ 258.0	$ 258.0
Accumulated Amortization	(44.0)	(32.8)
Net Amount	214.0	225.2
Aggregate intangible asset amortization	11.2	11.4	$ 9.1
Estimated amortization expenses related to intangibles
2019	11.0
2020	11.0
2021	10.9
2022	10.7
2023	10.7
Thereafter	132.4
Net Amount	186.7
Ocean Transportation | Customer Relationships.
Intangible assets
Gross Amount	140.6	140.6
Accumulated Amortization	(24.4)	(17.8)
Net Amount	$ 116.2	122.8
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	(19.6)	(15.0)
Net Amount	97.8	102.4
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	(19.6)	(15.0)
Net Amount	70.5	$ 75.1
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