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Stock Transactions and Unit Redemptions - Changes in Shares of Class A Common Stock Outstanding (Parenthetical) (Detail) - shares shares in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Limited Partnership [Member] |  |
Class of Stock [Line Items] |  |
Number of units redeemed and cancelled for exchange | 9.2 | 11.5
Class A Common Stock [Member] |  |
Class of Stock [Line Items] |  |
Common stock, shares issued | 9.5 | 10.1

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