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Derivative Instruments (Narrative) (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Derivative [Line Items]    
Derivative collateral $ 800.0 $ 1,600.0
Credit risk, maximum exposure $ 20.8 $ 11.9
Call Options [Member]    
Derivative [Line Items]    
Derivative, term of contract 1 year  
Interest Rate Swap [Member]    
Derivative [Line Items]    
Derivative, term of contract 7 years  
Interest Rate Caps [Member]    
Derivative [Line Items]    
Derivative, term of contract 7 years  
Interest Rate Caps [Member] | London Interbank Offered Rate (LIBOR) [Member]    
Derivative [Line Items]    
Derivative, cap interest rate 2.50%