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Fair Value Measurements (Tables)
3 Months Ended
Mar. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]  
Valuation of senior notes
 
March 31, 2014
 
December 31, 2013
 
Carrying Amount
 
Estimated Fair Value
 
Carrying Amount
 
Estimated Fair Value
4.875% Senior Notes due 2021
$
497,613

 
$
535,331

 
$
497,525

 
$
524,434

4.625% Senior Notes due 2021
$
299,611

 
$
320,329

 
$
299,598

 
$
310,141

3.800% Senior Notes due 2022
$
299,035

 
$
295,136

 
$
299,011

 
$
290,453

3.850% Senior Notes due 2023
$
299,977

 
$
295,485

 
$
299,976

 
$
289,362