Stockholders' Deficit - Narrative (Details) $ / shares in Units, $ in Millions		3 Months Ended			6 Months Ended		12 Months Ended
	Jul. 30, 2025 USD ($)	Sep. 30, 2025 USD ($) vote $ / shares	Jun. 30, 2025 $ / shares	Sep. 30, 2024 USD ($) $ / shares	Sep. 30, 2025 USD ($) vote $ / shares	Sep. 30, 2024 USD ($) $ / shares	Mar. 31, 2025 USD ($)
Dividends Payable [Line Items]
Number of votes per share of common stock permitted on proposals presented to stockholders (vote) | vote		1			1
Cash dividends declared, per common share (in dollars per share) | $ / shares		$ 0.82	$ 0.71	$ 0.71	$ 1.53	$ 1.33
Excise taxes					$ 10		$ 26
Excise tax payment	$ 26
Share Repurchase Transactions
Dividends Payable [Line Items]
Excise taxes		$ 8		$ 15	$ 10	$ 16