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Stock Transactions and Unit Redemptions - Changes in Shares of Class A Common Stock Outstanding Table Footnote (Detail) - shares shares in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Class A Common Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Common stock, shares issued (in shares) | 5.8 | 0.5 | 21.2 | 2.6
Limited Partnership |  |  |  |
Class of Stock [Line Items] |  |  |  |
Number of units redeemed and cancelled for exchange (in shares) | 6.9 | 0.4 | 23.2 | 2.5

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