SIGNIFICANT ACCOUNTING POLICIES - INVESTMENT IN SSAT (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Investments in affiliates
Investment in SSAT	$ 37.2		$ 37.2		$ 48.7
Financial information for equity method investment
Company Share of Net Income	13.0	$ 7.7	35.0	$ 15.4
Ocean Transportation
Financial information for equity method investment
Company Share of Net Income	13.0	7.7
SSAT
Investments in affiliates
Investment in SSAT	37.2		37.2		$ 48.7
Financial information for equity method investment
Operating revenue	316.3	273.9	944.6	796.1
Operating costs and expenses	(274.2)	(243.1)	(831.3)	(738.4)
Operating Income	42.1	30.8	113.3	57.7
Net Income	37.3	26.2	100.5	51.5
Company Share of Net Income	$ 13.0	$ 7.7	$ 35.0	$ 15.4