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Stock Transactions and Unit Redemptions - Additional Information (Detail) - USD ($)
 1 Months Ended 3 Months Ended 12 Months Ended
Mar. 09, 2018
Apr. 12, 2017
May 01, 2018
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Feb. 07, 2017
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       $ 172,167,800      
Redemptions, Number of Units       1,723,654      
Class B Common Stock [Member]              
Class of Stock [Line Items]              
Common stock, issued during the period       0 0    
Common stock outstanding       34,848,107 34,848,107 34,848,000  
Common stock, shares issued       34,848,000   34,848,000  
Common stock value issued       $ 348,000   $ 348,000  
Class A Common Stock [Member]              
Class of Stock [Line Items]              
Common stock, issued during the period       18,925,702   1,994,670  
Common stock outstanding       279,279,000   256,968,000  
Common stock, shares issued       328,529,000   306,218,000  
Common stock value issued       $ 3,286,000   $ 3,063,000  
Company's repurchase and additional redemption authority amount             $ 300,000,000
Stock repurchase program and unit redemption remaining authorized amount       $ 172,200,000      
Class A Common Stock [Member] | April 2017 Sales Agreement              
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, issued during the period     3,200,000        
Common stock, shares issued       20,000,000      
Redemptions, Number of Units       1,700,000      
Class A Common Stock [Member] | March 2019 Sales Agreement              
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       2,653,971      
Common stock value issued       $ 37,100