Debt (MARAD Debt) (Details) - USD ($) $ in Millions	Dec. 31, 2024	Sep. 30, 2024
Bonds | MARAD debt
Debt Instrument [Line Items]
Other long-term debt	$ 174
Bonds | MARAD debt - 3.432% bonds due 2036
Debt Instrument [Line Items]
Other long-term debt	55
Bonds | MARAD debt - 3.477% bonds due 2037
Debt Instrument [Line Items]
Other long-term debt	57
Bonds | MARAD debt - 3.609% bonds due 2038
Debt Instrument [Line Items]
Other long-term debt	$ 62
Coastal Holdings
Debt Instrument [Line Items]
Equity method investments, ownership percentage		50.00%