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Derivative Instruments, Hedging Activities and Fair Value Measurements, Other Fair Value Measurements (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Carrying Value [Member]    
Financial Liabilities: [Abstract]    
Fixed-rate debt obligations $ 21,480 $ 20,850
Level 2 [Member] | Fair Value [Member]    
Financial Liabilities: [Abstract]    
Fixed-rate debt obligations $ 23,470 $ 21,950