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Revolving Credit Facilities - Information Regarding Company's Revolving Credit Facilities (Detail) - Revolving Credit Facility [Member] - Unsecured Debt [Member]
$ in Millions
6 Months Ended
Jun. 30, 2017
USD ($)
JP Morgan Securities, LLC and Wells Fargo Securities, LLC [Member]  
Debt Instrument [Line Items]  
Carrying Value $ 0
Lines of Credit Maturity Date 2019-06
PNC Bank National Association [Member]  
Debt Instrument [Line Items]  
Carrying Value $ 0
Lines of Credit Maturity Date 2019-06