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Fair Value Measurements (Details) - USD ($)
Dec. 31, 2015
Mar. 31, 2015
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Carrying amount of liabilities $ 8,711,000,000 $ 9,709,000,000  
Transfers between level 1, level 2, or level 3 of the fair value hierarchy 0   $ 0
Fair malue, measurements, recurring | Fair value, inputs, level 2      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Estimated fair values of liabilities 9,000,000,000 10,400,000,000  
Fair malue, measurements, recurring | Fair value, inputs, level 1      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments in money market funds, time deposits and repurchase agreements $ 1,900,000,000 $ 4,200,000,000