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Derivative Instruments (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Mar. 09, 2017
Derivative Instruments      
Net Notional Amount of Interest Rate Swaps     $ 400.0
Interest Rate Swap [Member]      
Derivative Instruments      
Net Notional Amount of Interest Rate Swaps $ 1,740.8    
Weighted average fixed leg (Pay) interest rate (as a percent) 1.70%    
Interest Rate Swaps Average Remaining Maturity 3 years 3 months    
Interest Rate Cap [Member]      
Derivative Instruments      
Net Notional Amount of Interest Rate Swaps   $ 69.2  
Cash Flow Hedges | Designated as Hedging Instrument [Member]      
Derivative Instruments      
Unrealized gain on derivatives $ 6,115.0