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Financing arrangements (Shelf Registration Statements and Other Facilities) (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2014
Debt Instrument [Line Items]      
Additional period for extension of commitment term 1 year    
Line of credit [Member]      
Debt Instrument [Line Items]      
Maximum current borrowing capacity under a syndicated, unsecured, revolving credit agreement     $ 2,500,000,000
Amount by which borrowing capacity under a syndicated unsecured revolving credit agreement may be increased upon our request at discretion of banks     $ 500,000,000
Initial commitment term of each bank which is a party to the agreement     5 years
Additional period for extension of commitment term   1 year  
Annual commitment fees for syndicated, unsecured, revolving credit agreement 0.09%    
Basis spread on variable rate 1.00%    
Amount outstanding under syndicated, unsecured, revolving credit facility $ 0 $ 0