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v3.21.1

Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Millions |  | 1 Months Ended | 3 Months Ended | 6 Months Ended |
--- | --- | --- | --- | --- | ---
Feb. 28, 2020 $ / shares shares | Mar. 31, 2021 USD ($) shares | Feb. 28, 2021 shares | Dec. 31, 2020 shares | Mar. 31, 2021 USD ($) shares | Mar. 31, 2021 USD ($) shares | Mar. 31, 2020 USD ($) | 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 |  |  |  |  |  |  |
Non-cash stock-based compensation expense | $ |  |  |  |  | $ 15 | $ 21 | $ 160 |
Stock-based compensation expense | $ |  |  |  |  | 11 | 17 |  |
Share-based compensation liability | $ |  | $ 4 |  |  | 4 | 4 |  |
Additional Paid-in Capital |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Stock-based compensation expense | $ |  |  |  |  | $ 11 | $ 17 |  |
Restricted Stock Units (RSUs) |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Shares issued under Omnibus Incentive Plan (in shares) |  |  |  |  |  | 61,423 |  |
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) |  |  |  |  | 30,509 | 33,062 |  |
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 |  | 1,000,000,000
Common stock outstanding (in shares) |  | 116,921,411 |  |  | 116,921,411 | 116,921,411 |  | 88,578,361
Shares issued in period (in shares) |  |  |  | 4,321,259 |  |  |  |
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 | 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 | 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 |  | 1,000,000,000
Common stock outstanding (in shares) | 510,000,000 | 397,461,268 |  |  | 397,461,268 | 397,461,268 |  | 421,450,000
Exchange of LLC units for class B shares (in shares) |  | 968,920 |  | 18,265,183 |  |  |  |
Conversion of Class B shares (in shares) |  |  | 4,754,626 |  |  |  |  |
Class B Common Stock | IPO |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Common stock outstanding (in shares) | 421,450,000 |  |  |  |  |  |  |

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