Short-Term Borrowings (Narrative) (Details)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2015
USD ($)
$ / shares
Dec. 31, 2014
USD ($)
Dec. 05, 2014
USD ($)
Short-term Debt [Line Items]      
Line of Credit Facility, Maximum Borrowing Capacity $ 2,600.0    
Default Provision Floor 50.0    
Letters Of Credit      
Short-term Debt [Line Items]      
Letters of Credit Outstanding, Amount 0.0    
Revolving Credit Facility      
Short-term Debt [Line Items]      
Line of Credit Facility, Amount Outstanding $ 15.0 $ 0.0  
Federal Funds Effective Swap Rate [Member]      
Short-term Debt [Line Items]      
Debt Instrument, Basis Spread on Variable Rate 0.50%    
London Interbank Offered Rate (LIBOR)      
Short-term Debt [Line Items]      
Debt Instrument, Basis Spread on Variable Rate 1.00%    
Minimum      
Short-term Debt [Line Items]      
Line of Credit Facility, Commitment Fee Percentage 0.125%    
Distribution Made to Limited Partner, Distributions Paid, Per Unit | $ / shares $ 0.1675    
Minimum | London Interbank Offered Rate (LIBOR)      
Short-term Debt [Line Items]      
Margin Rate 0.00%    
Minimum | Eurodollar [Member]      
Short-term Debt [Line Items]      
Margin Rate 1.00%    
Maximum      
Short-term Debt [Line Items]      
Line of Credit Facility, Commitment Fee Percentage 0.35%    
Maximum | London Interbank Offered Rate (LIBOR)      
Short-term Debt [Line Items]      
Margin Rate 0.65%    
Maximum | Eurodollar [Member]      
Short-term Debt [Line Items]      
Margin Rate 1.65%    
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    
Period 3 | Maximum      
Short-term Debt [Line Items]      
Ratio of Indebtedness to Net Capital 1.00    
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    
Letters Of Credit      
Short-term Debt [Line Items]      
Line of Credit Facility, Maximum Borrowing Capacity $ 50.0    
Revolving Credit Facility      
Short-term Debt [Line Items]      
Line of Credit Facility, Maximum Borrowing Capacity $ 500.0   $ 1,500.0