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Revolving Credit Facilities - Information Regarding Company's Revolving Credit Facilities (Detail) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Carrying Value $ 60,000 $ 0
Revolving Credit Facility [Member] | Unsecured Debt [Member] | J.P. Morgan Chase Bank, N.A. and Wells Fargo Securities, LLC [Member]    
Debt Instrument [Line Items]    
Carrying Value $ 60,000  
Weighted-Average Interest Rate 2.20%  
Lines of Credit Maturity Date 2021-09  
Revolving Credit Facility [Member] | Unsecured Debt [Member] | PNC Bank National Association [Member]    
Debt Instrument [Line Items]    
Carrying Value $ 0  
Lines of Credit Maturity Date 2021-09