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Stock Transactions and Unit Redemptions - Gross Unit Redemptions and Share Repurchases of Class A Common Stock (Parenthetical) (Detail) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2020 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Class of Stock [Line Items] |  |  |  |
Weighted-average price for redemption of limited partnership units |  | $ 3.03 | $ 5.92 |
Repurchase of Class A common stock, shares Treasury Stock Shares Acquired | 2,000 | 2,000 |  |
Aggregate purchase price of Class A common stock |  | $ 6,000 | $ 1,236,000 | $ 10,439,000
Class A Common Stock [Member] |  |  |  |
Class of Stock [Line Items] |  |  |  |
Common stock, shares issued for redemption/cancellation of units |  | 9,500,000 | 10,100,000 |
Common stock, shares issued for exchange of units |  | 3,700,000 | 4,400,000 |
Repurchase of Class A common stock, shares Treasury Stock Shares Acquired |  | 2,259 | 233,172 | 788,788
Aggregate purchase price of Class A common stock |  | $ 6,000 | $ 1,200,000 |
Weighted-average price of Class A common stock |  | $ 2.58 | $ 5.30 |
Limited Partnership [Member] |  |  |  |
Class of Stock [Line Items] |  |  |  |
Redeemed limited partnership units |  | 1,800,000 | 1,400,000 |
Aggregate redemption price of limited partnership units |  | $ 5,500,000 | $ 8,000,000.0 |
Other Ownership Interest [Member] |  |  |  |
Class of Stock [Line Items] |  |  |  |
Redeemed limited partnership units |  | 700,000 | 56,900 |
Aggregate redemption price of limited partnership units |  | $ 1,300,000 | $ 320,600 |
Weighted-average price for redemption of founding/working partner units |  | $ 1.79 | $ 5.64 |

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- Definition Total change in each class of partners' capital accounts during the year due to redemptions and adjustments to redemption value. All partners include general, limited and preferred partners. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 5 -Subparagraph (SAB Topic 4.F) -URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187171-122770 + Details
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