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Stock Transactions and Unit Redemptions - Changes in Shares of Class A Common Stock Outstanding (Parenthetical) (Detail) - shares shares in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019
Class A Common Stock [Member] |  |  |  |
Class of Stock [Line Items] |  |  |  |
Common stock, shares issued | 0.7 | 2.3 | 2.1 | 2.4
Limited Partnership [Member] |  |  |  |
Class of Stock [Line Items] |  |  |  |
Number of units redeemed and cancelled for exchange | 0.6 | 2.5 | 2.1 | 2.6

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