SIGNIFICANT ACCOUNTING POLICIES - INVESTMENT IN SSAT (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Investments in affiliates
Investment in Terminal Joint Venture	$ 55.2		$ 55.2		$ 76.2
Financial information for equity method investment
Current assets	289.6		289.6		289.6
Noncurrent assets	2,517.1		2,517.1		2,555.8
Total Assets	2,806.7		2,806.7		2,845.4
Current liabilities	453.6		453.6		436.7
Noncurrent liabilities	1,461.3		1,461.3		1,603.0
Total liabilities and shareholders' equity	2,806.7		2,806.7		2,845.4
Company Share of Net Income	7.7	$ 8.4	15.4	$ 17.8
Ocean Transportation
Financial information for equity method investment
Company Share of Net Income	7.7	8.4	15.4	17.8
SSAT
Investments in affiliates
Increase in investment in SSAT			2.2
Investment in Terminal Joint Venture	55.2		55.2		$ 76.2
Financial information for equity method investment
Operating revenue	273.9	288.7	796.1	825.6
Operating costs and expenses	(243.1)	(262.6)	(738.4)	(771.8)
Operating Income	30.8	26.1	57.7	53.8
Net Income	$ 26.2	$ 23.3	$ 51.5	$ 50.1