INVESTMENT IN SSAT (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
	Mar. 01, 2024	Sep. 30, 2025	Sep. 30, 2024	Mar. 31, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Investments in affiliates
Investment in SSAT		$ 107.2			$ 107.2		$ 84.1
Financial information for equity method investment
Company's Share of SSAT's Net Income (Loss)		9.3	$ 6.9		23.2	$ 8.5
SSAT ST
Investments in affiliates
Ownership percentage, noncontrolling owner	50.00%
SSAT
Investments in affiliates
Investment in SSAT		107.2			107.2		$ 84.1
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		329.5	320.8		948.7	865.0
Operating costs and expenses		(311.8)	(301.3)		(903.9)	(858.2)
Operating income (loss)		17.7	19.5		44.8	6.8
SSAT's Net Income (Loss)		22.8	21.6		59.2	19.9
SSAT
Financial information for equity method investment
Company's Share of SSAT's Net Income (Loss)		$ 9.3	$ 6.9		$ 23.2	$ 8.5
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%