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Stock Transactions and Unit Redemptions - Additional Information (Detail) - USD ($)
 3 Months Ended
Mar. 09, 2018
Apr. 12, 2017
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Aug. 01, 2018
Class of Stock [Line Items]            
Payment percentage of the gross proceeds from the sale of shares   2.00%        
Stock repurchase program and unit redemption remaining authorized amount     $ 257,796,140      
Class B Common Stock [Member]            
Class of Stock [Line Items]            
Common stock, issued during the period     0 0    
Common stock outstanding     45,884,380   45,884,380  
Common stock, shares issued     45,884,000   45,884,000  
Common stock value issued     $ 459,000   $ 459,000  
Class A Common Stock [Member]            
Class of Stock [Line Items]            
Common stock, issued during the period     61,642 18,925,702    
Common stock outstanding     293,382,000   291,475,000  
Common stock, shares issued     343,885,000   341,745,000  
Common stock value issued     $ 3,439,000   $ 3,417,000  
Company's repurchase and additional redemption authority amount           $ 300,000,000
Stock repurchase program and unit redemption remaining authorized amount     $ 257,800,000      
Class A Common Stock [Member] | April 2017 Sales Agreement [Member]            
Class of Stock [Line Items]            
Maximum number of shares under the controlled equity offering   20,000,000        
Class A Common Stock [Member] | Controlled Equity Offerings [Member]            
Class of Stock [Line Items]            
Common stock, shares issued     20,000,000      
Class A Common Stock [Member] | March 2018 Sales Agreement [Member]            
Class of Stock [Line Items]            
Maximum number of shares under the controlled equity offering 300,000,000          
Class A Common Stock [Member] | New Controlled Equity Offerings [Member]            
Class of Stock [Line Items]            
Common stock, shares issued     17,401,431      
Common stock value issued     $ 209,800,000