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Fair Value Measurements (Tables)
3 Months Ended
Mar. 31, 2013
Disclosure Text Block  
Fair value measurements
   Level within the Fair Value Hierarchy March 31, 2013 December 31, 2012
    Book Value Fair Value Book Value Fair Value
(Dollars in thousands)             
Cash and cash equivalents1 $ 766,700 $ 766,700 $ 740,481 $ 740,481
Short-term investments             
 Government-backed securities1   125,595   125,595   115,700   115,700
Long-term investments             
 Government-backed securities1   40,142   40,175   50,305   50,339
Long-term debt             
 Retail1   1,178,250   1,230,244   1,178,250   1,238,204
 Institutional and other2   538,369   591,696   538,657   589,435