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Derivatives and Hedging Activities Derivatives in Cash Flow Hedging Relationships (Interest Rate Swaps) (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
Debt_Instrument
Dec. 31, 2016
USD ($)
Debt_Instrument
Dec. 31, 2015
USD ($)
Derivative Instruments, Gain (Loss) [Line Items]      
Amount of gain (loss) recognized in OCI on derivatives $ 2,770 $ 6,979 $ (3,128)
Amount of loss reclassified from AOCI into interest expense 1,810 $ 3,586 $ 3,150
Interest rate derivative liabilities, at fair value 100    
Designated as Hedging Instrument      
Derivative Instruments, Gain (Loss) [Line Items]      
Reversal of cumulative ineffectiveness gain due to adoption of new accounting pronouncement 1,300    
Interest Rate Swap | Designated as Hedging Instrument      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative instruments, gain (loss) reclassification from AOCI to income, estimated net amount to be transferred $ 1,500    
Count | Debt_Instrument 6 4  
Notional amount $ 992,000 $ 642,000  
Future | Designated as Hedging Instrument      
Derivative Instruments, Gain (Loss) [Line Items]      
Count | Debt_Instrument   1  
Notional amount   $ 255,000  
Minimum | Interest Rate Swap | Designated as Hedging Instrument      
Derivative Instruments, Gain (Loss) [Line Items]      
Fixed LIBOR 1.20% 1.20%  
Maximum | Interest Rate Swap | Designated as Hedging Instrument      
Derivative Instruments, Gain (Loss) [Line Items]      
Fixed LIBOR 2.20% 1.50%