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Interest Rate Caps -Additional Information (Detail) - USD ($)
6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Derivative [Line Items]    
Notional amount of interest rate cap $ 400,000,000  
London Interbank Offered Rate (LIBOR) [Member]    
Derivative [Line Items]    
Interest rate 2.09% 1.56%
Interest Rate Cap Contracts Expiring in March 2019 [Member]    
Derivative [Line Items]    
Notional amount of interest rate cap $ 50,000,000  
Maturity period of interest rate caps exchanged Mar. 31, 2019  
Strike rate 2.50%  
Interest Rate Cap Contracts Expiring In April 2020 [Member]    
Derivative [Line Items]    
Notional amount of interest rate cap $ 100,000,000  
Maturity period of interest rate caps exchanged Apr. 30, 2020  
Strike rate 3.25%  
Interest Rate Cap Contracts Expiring In June 2020 [Member]    
Derivative [Line Items]    
Notional amount of interest rate cap $ 50,000,000  
Maturity period of interest rate caps exchanged Jun. 30, 2020  
Strike rate 2.50%  
Interest Rate Cap Contracts Expiring In April 2021 [Member]    
Derivative [Line Items]    
Notional amount of interest rate cap $ 200,000,000  
Maturity period of interest rate caps exchanged Apr. 30, 2021  
Strike rate 3.50%