Consolidated Statements of Comprehensive Income (Loss) (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Statement of Comprehensive Income [Abstract]
Net derivative gains (losses) and other costs arising during the period, net of tax expense (benefit)	$ 9.5	$ 1.7	$ 0.8
Reclassification of (gains) losses into net income (loss), net of tax (expense) benefit	$ (3.8)	$ (0.3)	$ (1.2)