Income Taxes - Valuation Allowances (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Valuation Allowance [Line Items]
Deferred tax assets, impairment of equity securities and other unrealized investment losses.	$ 824
Impairment of equity securities
Valuation Allowance [Line Items]
Deferred tax assets, valuation allowance	$ 788	$ 635

Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482685/740-10-50-2

Name:	us-gaap_DeferredTaxAssetsValuationAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant