INVESTMENT IN SSAT (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Mar. 01, 2024	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Investments in affiliates
Investment in SSAT		$ 86.4		$ 86.4		$ 85.5
Financial information for equity method investment
Company Share of SSAT's Net Income (Loss) (2)		1.2	$ (1.4)	1.6	$ (3.2)
SSAT ST
Investments in affiliates
Ownership percentage, noncontrolling owner	50.00%
SSAT
Investments in affiliates
Investment in SSAT		86.4		86.4		$ 85.5
Increase in equity method investment in SSAT	$ 13.2
Equity Method Investments, Increase (decrease) in deferred income taxes expense (benefit),	3.1
Equity method investments, increase (decrease) in retained earnings	$ 10.1
Financial information for equity method investment
Operating revenue		273.9	242.9	544.2	473.7
Operating costs and expenses		(278.5)	(248.9)	(556.9)	(484.8)
Operating loss		(4.6)	(6.0)	(12.7)	(11.1)
Net Profit (Loss) (1)		2.1	(4.1)	(1.7)	(8.9)
SSAT
Financial information for equity method investment
Company Share of SSAT's Net Income (Loss) (2)		$ 1.2	$ (1.4)	$ 1.6	$ (3.2)
SSAT | SSAT ST
Investments in affiliates
Percentage of interests sold in SSAT ST	25.00%
SSAT's ownership interest in SSAT ST (as a percent)	50.00%