GOODWILL AND INTANGIBLE ASSETS - GOODWILL (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Changes in the Company's goodwill
Balance at the beginning of the period	$ 323.7	$ 241.6
Additions		0.0
Balance at the end of the period	323.7	323.7
Ocean Transportation
Changes in the Company's goodwill
Balance at the beginning of the period	218.5	215.0
Balance at the end of the period	218.5	218.5
Logistics
Changes in the Company's goodwill
Balance at the beginning of the period	105.2	26.6
Balance at the end of the period	105.2	105.2
Span Alaska
Changes in the Company's goodwill
Additions		78.6
Span Alaska | Ocean Transportation
Changes in the Company's goodwill
Additions		0.0
Span Alaska | Logistics
Changes in the Company's goodwill
Additions		78.6
Horizon
Changes in the Company's goodwill
Balance at the beginning of the period		217.7
Additions	0.0	3.5
Horizon | Ocean Transportation
Changes in the Company's goodwill
Additions	0.0	3.5
Horizon | Logistics
Changes in the Company's goodwill
Additions	$ 0.0	$ 0.0