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Derivatives - Interest Rate Risk (Narratives) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Derivatives        
Interest expense $ 22,504 $ 21,177 $ 43,925 $ 41,477
Deferred pre-tax losses to be reclassified during next 12 months $ 2,000   $ 2,000  
Derivatives designated as hedging instruments: | Interest rate swaps        
Derivatives        
Weighted average effective fixed interest rate on interest rate swaps (as a percent) 1.60%   1.60%  
Interest expense     $ 100