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Credit Facility - Additional Information (Detail) - Syndicated Facility Agreement [Member] - USD ($)
Jun. 19, 2015
Sep. 30, 2015
Line of Credit Facility [Line Items]    
Unsecured revolving line of credit facility $ 1,200,000,000  
Line of credit facility amount option to increase additional borrowings $ 350,000,000  
Credit facility agreement amendment date Jun. 19, 2015  
Outstanding line of credit   $ 125,000,000
Credit facility agreement, maturity date Jun. 19, 2020