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Fair Value Measurements (Table)
3 Months Ended
Mar. 31, 2015
Fair Value Disclosures  
Fair value measurements
  Level within the Fair Value Hierarchy March 31, 2015 December 31, 2014
   Book Value  Fair Value Book Value Fair Value
(Dollars in thousands)             
Cash and cash equivalents1 $ 336,893 $ 336,893 $ 211,513 $ 211,513
Long-term debt             
 Retail2   617,000   625,855   617,000   608,462
 Institutional2   532,793   526,199   532,722   513,647