Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Millions		3 Months Ended		6 Months Ended
	Feb. 28, 2020 $ / shares shares	Mar. 31, 2022 USD ($) shares	Mar. 31, 2021 USD ($) shares	Mar. 31, 2022 USD ($) shares	Mar. 31, 2021 USD ($) shares	Dec. 31, 2021 shares	Sep. 30, 2021 shares	Dec. 31, 2020 shares	Sep. 30, 2020 shares
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Capital stock authorized (in shares)	2,100,000,000
Preferred stock authorized (in shares)	100,000,000
Preferred stock par value (in dollars per share) | $ / shares	$ 1.00
Stock split, conversion ratio	477,242.614671815
Preferred stock outstanding (in shares)	0
Award requisite service period				4 years
Non-cash stock-based compensation expense | $		$ 6	$ 15	$ 30	$ 21
Share-based compensation liability | $		4		4
Additional Paid-in Capital
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Non-cash stock-based compensation expense | $		4		26
Share-based compensation liability | $		$ 9		$ 9
Restricted Stock Units (RSUs)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shares issued under Omnibus Incentive Plan (in shares)				527,565
Restricted Stock Units (RSUs) | Omnibus Incentive Plan
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shares issued under Omnibus Incentive Plan (in shares)		311,952	30,509	462,998	33,062
Restricted Stock
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Award requisite service period				1 year
IPO
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Award requisite service period				1 year
Class A Common Stock
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock authorized (in shares)	1,000,000,000	1,000,000,000		1,000,000,000			1,000,000,000
Common stock outstanding (in shares)		137,198,372		137,198,372			122,414,827
Class A Common Stock | Common Stock
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock outstanding (in shares)		137,198,372	116,921,411	137,198,372	116,921,411	127,236,783	122,414,827	111,167,356	88,578,361
Shares issued under Omnibus Incentive Plan (in shares)		35,387	30,509	186,433	33,062
Vesting of restricted stock units, net of shares withheld for employee taxes (in shares)		276,565		276,565
Conversion of Class B shares for Class A shares (in shares)		9,649,637	4,754,626	14,320,547	4,754,626
Exchange of LLC units for class B shares (in shares)			968,920		19,234,103
Class A Common Stock | Omnibus Incentive Plan
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock available for issuance (in shares)		31,169,099		31,169,099
Incentive plan period				10 years
Class A Common Stock | IPO
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock outstanding (in shares)	88,550,000
Class A Common Stock | IPO | Omnibus Incentive Plan
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock available for issuance (in shares)		1,000,000		1,000,000
Class B Common Stock
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock authorized (in shares)	1,000,000,000	1,000,000,000		1,000,000,000			1,000,000,000
Common stock outstanding (in shares)	510,000,000	377,650,449		377,650,449			391,970,996
Conversion of Class B shares for Class A shares (in shares)				13,810,382
Exchange of LLC units for class B shares (in shares)				510,165
Class B Common Stock | Common Stock
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock outstanding (in shares)		377,650,449	397,461,268	377,650,449	397,461,268	387,300,086	391,970,996	403,184,814	421,450,000
Conversion of Class B shares for Class A shares (in shares)		9,649,637	4,754,626	14,320,547	4,754,626
Exchange of LLC units for class B shares (in shares)			968,920		19,234,106
Class B Common Stock | IPO
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock outstanding (in shares)	421,450,000