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Fair Value Measurements (Tables)
9 Months Ended
Sep. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]  
Valuation of senior notes
 
September 30, 2014
 
December 31, 2013
 
Carrying Amount
 
Estimated Fair Value
 
Carrying Amount
 
Estimated Fair Value
4.875% Senior Notes due 2021
$
497,788

 
$
548,188

 
$
497,525

 
$
524,434

4.625% Senior Notes due 2021
$
299,637

 
$
325,432

 
$
299,598

 
$
310,141

3.800% Senior Notes due 2022
$
299,084

 
$
307,882

 
$
299,011

 
$
290,453

3.850% Senior Notes due 2023
$
299,979

 
$
307,859

 
$
299,976

 
$
289,362