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Fair Value of Financial Instruments (Narrative) (Details) - USD ($)
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Trading securities $ 203,000,000 $ 228,000,000
Default funds and margin deposits 3,633,000,000 3,301,000,000
Highly rated government debt securities 1,610,000,000 1,763,000,000
Transfers from between level 1 and level 2 0 0
Fair value of debt utilizing discounted cash flow analyses 3,800,000,000  
Reverse Repurchase Agreements    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Default funds and margin deposits 576,000,000 137,000,000
Foreign Government Debt Securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Trading securities $ 154,000,000 $ 172,000,000