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Fair Value Measurements Narrative (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Fair Value Measurements [Abstract]    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net $ 0  
Debt obligations, fair value 42,520 $ 45,620
Long-term Debt $ 42,190 $ 44,080