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Fair Value of Financial Instruments (Tables)
6 Months Ended
Jun. 30, 2012
Fair Value of Financial Instruments and Fair Value Measurement [Abstract]  
Carrying values and fair values of the Company's debt instruments
                                 
    Carrying Value at     Fair Value* at  
    June 30,
2012
    December 31,
2011
    June 30,
2012
    December 31,
2011
 

6.22% Senior Notes due 2015

  $ 175,000     $ 175,000     $ 196,605     $ 186,305  

3.30% Senior Notes due 2016

  $ 100,000     $ 100,000     $ 102,799     $ 98,980  

4.00% Senior Notes due 2018

  $ 50,000     $ 50,000     $ 52,501     $ 51,220  

5.25% Senior Notes due 2019

  $ 175,000     $ 175,000     $ 192,868     $ 174,125  

4.64% Senior Notes due 2021

  $ 100,000     $ 100,000     $ 103,804     $ 104,250