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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
Mar. 31, 2020
Mar. 31, 2019
Class of Stock [Line Items]    
Weighted-average price for redemption of limited partnership units $ 4.30 $ 6.00
Repurchase of Class A common stock, shares Treasury Stock Shares Acquired   233,172
Aggregate purchase price of Class A common stock   $ 1,236,000
Class A Common Stock [Member]    
Class of Stock [Line Items]    
Common stock, shares issued for redemption/cancellation of units 1,400,000 100,000
Common stock, shares issued for exchange of units 600,000 900,000
Repurchase of Class A common stock, shares Treasury Stock Shares Acquired 0 233,000
Aggregate purchase price of Class A common stock   $ 1,200,000
Weighted-average price of Class A common stock   $ 5.30
Limited Partnership [Member]    
Class of Stock [Line Items]    
Redeemed limited partnership units 200,000 1,200,000
Aggregate redemption price of limited partnership units $ 1,000,000.0 $ 7,200,000
Other Ownership Interest [Member]    
Class of Stock [Line Items]    
Redeemed limited partnership units 0 2,300
Aggregate redemption price of limited partnership units   $ 14,100
Weighted-average price for redemption of founding/working partner units   $ 6.11