Low Income Housing Partnerships (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Investment Program, Proportional Amortization Method, Elected [Line Items]
Investment In affordable housing limited partnerships	$ 131.4	$ 121.8
Obligations related to investments in affordable housing limited partnerships	$ 74.3	$ 63.6
Affordable housing tax credits commitment, year to be paid	2027
Proportional amortization expense, amount, income statement location	INCOME TAX EXPENSE (BENEFIT)	INCOME TAX EXPENSE (BENEFIT)	INCOME TAX EXPENSE (BENEFIT)
Affordable housing tax credits, amount, income statement location	INCOME TAX EXPENSE (BENEFIT)	INCOME TAX EXPENSE (BENEFIT)	INCOME TAX EXPENSE (BENEFIT)
Proportional amortization expense, amount	$ 10.4	$ 10.1	$ 9.3
Proportional amortization expense, amount, cash flow statement location	Increase (Decrease) in Other Operating Assets	Increase (Decrease) in Other Operating Assets	Increase (Decrease) in Other Operating Assets
Affordable housing tax credits, amount	$ 12.6	$ 12.4	$ 11.6
Affordable housing tax credits, amount, cash flow statement location	Provision for deferred income taxes	Income taxes receivable, net	Income taxes receivable, net
Net income tax benefit, low income housing partnerships	$ 2.2	$ 2.3	$ 2.3
Impairment losses from the forfeiture or ineligibility of tax credits or other circumstances, amount	$ 0.0	$ 0.0	$ 0.0