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Debt - Additional Information (Detail) - USD ($)
$ in Millions
9 Months Ended
Feb. 12, 2018
Sep. 30, 2018
Dec. 31, 2017
Debt Instrument [Line Items]      
Mortgage loan outstanding balance   $ 366.6 $ 297.3
7.25% Unsecured Notes [Member]      
Debt Instrument [Line Items]      
Interest rate on loan 7.25%    
Borrowed amount $ 25.0    
Debt instrument maturity date   Feb. 15, 2021  
Debt instrument callable date   Feb. 15, 2019  
Notes face value 101.00%