Short-Term Borrowings (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Dec. 05, 2014
Short-term Debt [Line Items]    
Revolving credit facility $ 2,600  
Line of Credit Facility Amount Outstanding 0  
Letters of Credit Outstanding 0  
Minimum    
Short-term Debt [Line Items]    
Line of Credit Facility, Commitment Fee Percentage 0.125%  
Consolidated Interest Coverage Ratio 3.00  
Maximum    
Short-term Debt [Line Items]    
Line of Credit Facility, Commitment Fee Percentage 0.35%  
Ratio of Indebtedness to Net Capital 1.00  
Consolidated Interest Coverage Ratio 1.00  
Revolving Credit Facility    
Short-term Debt [Line Items]    
Revolving credit facility 1,500 500
Letter of Credit    
Short-term Debt [Line Items]    
Revolving credit facility   $ 50
Period 1 | Minimum    
Short-term Debt [Line Items]    
Ratio of Indebtedness to Net Capital 5.75  
Period 1 | Maximum    
Short-term Debt [Line Items]    
Ratio of Indebtedness to Net Capital 1.00  
Period 2 | Minimum    
Short-term Debt [Line Items]    
Ratio of Indebtedness to Net Capital 5.50  
Period 2 | Maximum    
Short-term Debt [Line Items]    
Ratio of Indebtedness to Net Capital 1.00  
Period 3 | Minimum    
Short-term Debt [Line Items]    
Ratio of Indebtedness to Net Capital 5.00  
Acquisition Period | Minimum    
Short-term Debt [Line Items]    
Ratio of Indebtedness to Net Capital 5.50  
Acquisition Period | Maximum    
Short-term Debt [Line Items]    
Ratio of Indebtedness to Net Capital 1.00  
Federal Funds Effective Swap Rate    
Short-term Debt [Line Items]    
Basis Spread on Variable Rate 0.50%  
Federal Funds Effective Swap Rate | Minimum    
Short-term Debt [Line Items]    
Margin Rate 0.00%  
Federal Funds Effective Swap Rate | Maximum    
Short-term Debt [Line Items]    
Margin Rate 0.65%  
London Interbank Offered Rate (LIBOR)    
Short-term Debt [Line Items]    
Basis Spread on Variable Rate 1.00%  
London Interbank Offered Rate (LIBOR) | Minimum    
Short-term Debt [Line Items]    
Margin Rate 0.00%  
London Interbank Offered Rate (LIBOR) | Maximum    
Short-term Debt [Line Items]    
Margin Rate 0.65%  
Eurodollar    
Short-term Debt [Line Items]    
Margin Rate 1.00%  
Eurodollar | Maximum    
Short-term Debt [Line Items]    
Margin Rate 1.65%