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Fair Value Measurements (Tables)
3 Months Ended
Mar. 31, 2012
Fair Value Measurements Tables [Abstract]  
Fair Value, Measurement Inputs, Disclosure [Text Block]
   Level 1 Level 2 Level 3 Total
          
 Assets:        
 Current derivative asset$ -$ 289,078$ -$ 289,078
          
 Liabilities:        
 Current derivative liability$ -$ 946$ -$ 946
Schedule of Fair Value of Financial Instruments
   March 31, 2012 December 31, 2011
   Carrying Estimated Carrying Estimated
   Amount Fair Value Amount Fair Value
 Long-Term Debt:        
 5.45% Notes due 2015, issued 2008$ 100,000$ 111,404$ 100,000$ 111,475
 7.31% Notes due 2016, issued 2009  62,000  74,628  62,000  74,817
 4.98% Notes due 2017, issued 2010  116,000  129,801  116,000  128,570
 5.92% Notes due 2018, issued 2008  200,000  232,991  200,000  231,091
 7.77% Notes due 2019, issued 2009  173,000  220,103  173,000  219,552
 5.50% Notes due 2020, issued 2010  207,000  231,531  207,000  229,423
 4.51% Notes due 2020, issued 2010  315,000  322,868  315,000  318,925
 5.60% Notes due 2022, issued 2010  87,000  94,266  87,000  94,165
 4.66% Notes due 2022, issued 2010  35,000  34,719  35,000  34,631
 5.85% Notes due 2025, issued 2010  90,000  98,936  90,000  99,022
 4.91% Notes due 2025, issued 2010  175,000  174,137  175,000  173,835
 Credit Facility  459,000  459,000  343,000  343,000
  $ 2,019,000$ 2,184,384$ 1,903,000$ 2,058,506