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Derivative Instruments - Additional Information (Details)
$ in Billions
Sep. 30, 2018
USD ($)
Dec. 31, 2017
contract
Derivative [Line Items]    
Lines of credit outstanding $ 1.7  
Interest rate swaps    
Derivative [Line Items]    
Number of interest rate swap contracts entered into | contract   5
Borrowings under derivative agreements $ 1.3  
Interest rate swaps | Minimum    
Derivative [Line Items]    
Fixed rate variable interest 0.896%  
Interest rate swaps | Maximum    
Derivative [Line Items]    
Fixed rate variable interest 2.212%