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Stock Transactions and Unit Redemptions - Schedule of Share Repurchase Activity (Detail) - Class A Common Stock [Member] - USD ($)
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2019
Jun. 30, 2019
Dec. 31, 2018
Class Of Stock [Line Items]      
Balance at beginning of period, Total Number of Shares Repurchased   50,000  
Repurchase of common stock, shares 1,613,032 1,613,032  
Balance at Ending of period, Total Number of Shares Repurchased 1,663,032 1,663,032 50,000
Repurchase, Average Price Paid per Unit $ 8.61 $ 8.65 $ 9.73
Total Number of Shares Purchased as Part of Publicly Announced Program 1,613,032 1,663,032 50,000
Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan $ 186,104 $ 185,618 $ 199,514