GOODWILL AND INTANGIBLE ASSETS - INTANGIBLE ASSETS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Finite-lived Intangible Assets
Gross Amount at the beginning of the year	$ 151.4	$ 10.4
Additions		1.0
Gross Amount at the end of the year	258.0	151.4	$ 10.4
Accumulated Amortization	(21.4)	(12.3)
Net Amount	236.6	139.1
Aggregate intangible asset amortization	9.1	4.4	1.3
Estimated amortization expenses related to intangibles
2017	11.3
2018	11.2
2019	11.0
2020	11.0
2021	10.9
Thereafter	153.9
Net Amount	209.3
Horizon
Finite-lived Intangible Assets
Additions		140.0
Span Alaska
Finite-lived Intangible Assets
Additions	106.6
Estimated amortization expenses related to intangibles
Net Amount	27.3
Span Alaska | Trade Names
Finite-lived Intangible Assets
Indefinite-Lived intangible asset acquired	27.3
Indefinite-Lived intangible asset	27.3
Customer Relationships. | Span Alaska
Finite-lived Intangible Assets
Finite-Lived intangible assets acquired	79.3
Estimated amortization expenses related to intangibles
Net Amount	$ 209.3
Expected useful life	20 years
Ocean Transportation
Finite-lived Intangible Assets
Gross Amount at the beginning of the year	$ 140.6	0.6
Gross Amount at the end of the year	140.6	140.6	0.6
Accumulated Amortization	(11.0)	(4.2)
Net Amount	129.6	136.4
Ocean Transportation | Horizon
Finite-lived Intangible Assets
Additions		140.0
Ocean Transportation | Customer Relationships. | Horizon
Finite-lived Intangible Assets
Finite-Lived intangible assets acquired		$ 140.0
Estimated amortization expenses related to intangibles
Expected useful life		21 years
Logistics
Finite-lived Intangible Assets
Gross Amount at the beginning of the year	10.8	$ 9.8
Additions		1.0
Gross Amount at the end of the year	117.4	10.8	$ 9.8
Accumulated Amortization	(10.4)	(8.1)
Net Amount	107.0	$ 2.7
Logistics | Span Alaska
Finite-lived Intangible Assets
Additions	$ 106.6