Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Statement of Comprehensive Income [Abstract]
Net Income	$ 79.1	$ 78.6
Other Comprehensive Income, net of taxes
Foreign currency translation adjustments	(39.0)	46.7
Unrealized gain on derivative instruments	1.2	0.5
Post-retirement benefit plans	0.1	0.0
Other comprehensive (loss) income	(37.7)	47.2
Comprehensive Income	41.4	125.8
Other comprehensive loss attributable to non-controlling interest	(6.5)	0.0
Comprehensive Income Attributable to GATX	$ 47.9	$ 125.8