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Schedule of Committed Credit Facilities (Detail)
$ in Millions
Dec. 31, 2017
USD ($)
Revolving Credit Facility Maturing 2018  
Line of Credit Facility [Line Items]  
Total Capacity $ 1,800
Outstanding Borrowings 0
Letters of Credit Issued 782
Available Capacity 1,018
Other Credit Facility [Member]  
Line of Credit Facility [Line Items]  
Total Capacity 3
Outstanding Borrowings 3
Letters of Credit Issued 0
Available Capacity $ 0