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Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Millions |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 03, 2020 USD ($) | Feb. 28, 2020 $ / shares shares | Jun. 30, 2020 USD ($) shares | Jun. 30, 2020 USD ($) shares | Jun. 30, 2020 USD ($) shares | Jun. 05, 2020 $ / shares shares | Mar. 31, 2020 USD ($) | Oct. 01, 2019 USD ($) | Sep. 30, 2019 USD ($) shares | Jun. 30, 2019 USD ($) | Mar. 31, 2019 USD ($) | Sep. 30, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Capital stock authorized (in shares) | shares |  | 2,100,000,000 |  |  |  |  |  |  |  |  |  |
Preferred stock authorized (in shares) | 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) | shares |  |  | 0 | 0 | 0 | 0 |  |  |  |  |  |
Decrease to accumulated deficit |  |  | $ (21) | $ (21) | $ (21) |  | $ (285) |  | $ (269) | $ (133) | $ (120) | $ (320)
Non-cash stock-based compensation expense |  |  |  | 437 | 437 |  |  |  |  |  |  |
Modification of stock-based compensation plan |  |  |  | 769 | 769 |  |  |  |  |  |  |
Lease liabilities |  |  | 346 | 346 | 346 |  |  |  |  |  |  |
Operating lease right-of-use assets |  |  | 278 | 278 | 278 |  |  |  |  |  |  |
Deferred rent liability |  |  |  |  |  |  |  |  | 69 |  |  |
Decrease to accumulated deficit |  |  | (1,685) | (1,685) | (1,685) |  |  |  | (1,177) |  |  |
Share-based Payment Arrangement |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock-based compensation expense |  |  | 3 |  |  |  |  |  |  |  |  |
Accumulated Deficit |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Decrease to accumulated deficit |  |  | (1,685) | (1,685) | (1,685) |  | (1,166) |  | (1,177) | (1,061) | (1,043) | (1,272)
Additional Paid-in Capital |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Decrease to accumulated deficit |  |  | 1,899 | 1,899 | 1,899 |  | 1,127 |  | 1,127 | $ 1,127 | $ 1,127 | 1,127
Modification of stock-based compensation plan | $ 769 |  |  | 769 | 769 |  |  |  |  |  |  |
Additional Paid-in Capital | IPO Tag-Along Right |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Modification of stock-based compensation plan | $ 57 |  |  |  |  |  |  |  |  |  |  |
Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Decrease to accumulated deficit |  |  |  |  |  |  | 0 |  |  |  |  | 150
Cumulative Effect, Period of Adoption, Adjustment | Accumulated Deficit |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Decrease to accumulated deficit |  |  |  |  |  |  | $ 0 |  |  |  |  | $ 139
ASC 718 |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Decrease to share-based compensation liability |  |  |  | 38 |  |  |  |  |  |  |  |
ASC 718 | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Decrease to accumulated deficit |  |  |  |  |  |  |  |  | 33 |  |  |
ASC 718 | Cumulative Effect, Period of Adoption, Adjustment | Accumulated Deficit |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Decrease to accumulated deficit |  |  | $ 33 | $ 33 | $ 33 |  |  |  | 33 |  |  |
ASC 842 |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Lease liabilities |  |  |  |  |  |  |  | $ 366 |  |  |  |
Operating lease right-of-use assets |  |  |  |  |  |  |  | 297 |  |  |  |
Decrease to accumulated deficit |  |  |  |  |  |  |  | $ 7 |  |  |  |
ASC 842 | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Decrease to accumulated deficit |  |  |  |  |  |  |  |  | 7 |  |  |
ASC 842 | Cumulative Effect, Period of Adoption, Adjustment | Accumulated Deficit |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Decrease to accumulated deficit |  |  |  |  |  |  |  |  | $ 7 |  |  |
Class A Common Stock |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common stock authorized (in shares) | 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) | shares |  |  | 77,000,000 | 77,000,000 | 77,000,000 | 77,000,000 |  |  | 0 |  |  |
Class A Common Stock | Omnibus Incentive Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common stock available for issuance (in shares) | shares |  |  | 31,169,099 | 31,169,099 | 31,169,099 |  |  |  |  |  |  |
Incentive plan period |  |  |  |  | 10 years |  |  |  |  |  |  |
Shares issued in period (in shares) | shares |  |  |  |  | 0 |  |  |  |  |  |  |
Class A Common Stock | IPO |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Price per share (in dollars per share) | $ / shares |  |  |  |  |  | $ 25 |  |  |  |  |  |
Class A Common Stock | IPO | Omnibus Incentive Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common stock available for issuance (in shares) | 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) | 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) | shares |  | 510,000,000 | 433,000,000 | 433,000,000 | 433,000,000 | 433,000,000 |  |  | 505,830,022 |  |  |

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