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Schedule of Net Amount Available Under Credit Facility (Detail) - Unsecured Credit Facility - USD ($)
Dec. 30, 2017
Nov. 29, 2017
Dec. 31, 2016
Debt Instrument [Line Items]      
Gross amount available $ 500,000,000 $ 700,000,000  
Outstanding     $ (24,000,000)
Letters of credit (8,698,000)    
Available for withdrawal $ 491,302,000