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Derivative Instruments (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]    
Derivative collateral $ 700.0 $ 200.0
Credit risk, maximum exposure $ 66.6 $ 16.1
London Interbank Offered Rate (LIBOR) | Interest Rate Caps    
Derivative [Line Items]    
Cap rate 2.50%