SIGNIFICANT ACCOUNTING POLICIES - INVESTMENT IN SSAT (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Investments in affiliates
Investment in Terminal Joint Venture	$ 50.1		$ 50.1		$ 48.7
Financial information for equity method investment
Company Share of Net Income	12.8	$ 3.7	22.0	$ 7.7
Ocean Transportation
Financial information for equity method investment
Company Share of Net Income	12.8	3.7
SSAT
Investments in affiliates
Investment in Terminal Joint Venture	50.1		50.1		$ 48.7
Financial information for equity method investment
Operating revenue	322.2	243.3	628.3	522.2
Operating costs and expenses	(284.0)	(229.9)	(557.1)	(495.3)
Operating Income	38.2	13.4	71.2	26.9
Net Income	34.3	12.3	63.2	25.3
Company Share of Net Income	$ 12.8	$ 3.7	$ 22.0	$ 7.7