GOODWILL AND INTANGIBLE ASSETS - INTANGIBLE ASSETS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Intangible assets
Gross Amount	$ 263.2	$ 258.0
Total Intangible Assets, net	174.9	181.1
Aggregate intangible asset amortization	11.4	10.9	$ 10.9
Less: Accumulated Amortization	(88.3)	(76.9)
Total estimated amortization expense related to intangibles	$ 147.6
Customer Relationships | Maximum
Intangible assets
Expected useful life	21 years
Ocean Transportation | Customer Relationships
Intangible assets
Gross Amount	$ 140.6	140.6
Total Intangible Assets, net	89.4	96.0
Less: Accumulated Amortization	$ (51.2)	(44.6)
Ocean Transportation | Customer Relationships | Horizon
Intangible assets
Expected useful life	21 years
Finite-Lived intangible assets acquired	$ 140.6
Logistics
Intangible assets
Gross Amount	122.6	117.4
Total Intangible Assets, net	85.5	85.1
Less: Accumulated Amortization	(37.1)	(32.3)
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	95.3	90.1
Total Intangible Assets, net	58.2	57.8
Net Remaining Amount	16.0
Less: Accumulated Amortization	$ (37.1)	$ (32.3)
Logistics | Customer Relationships | Minimum
Intangible assets
Expected useful life	3 years
Logistics | Customer Relationships | Maximum
Intangible assets
Expected useful life	13 years
Logistics | Customer Relationships | Span Alaska
Intangible assets
Total Intangible Assets, net	$ 79.3
Expected useful life	20 years