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Fair Value Measurements (Tables)
3 Months Ended
Mar. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]  
Valuation of marketable securities
 
March 31, 2019
 
Quoted Prices in Active Markets for Identical Instruments
(Level 1)
 
Significant Other Observable Inputs
(Level 2)
 
Significant Unobservable Inputs
(Level 3)
 
Total
Marketable securities
$
28,149

 
$

 
$

 
$
28,149


 
December 31, 2018
 
Quoted Prices in Active Markets for Identical Instruments
(Level 1)
 
Significant Other Observable Inputs
(Level 2)
 
Significant Unobservable Inputs
(Level 3)
 
Total
Marketable securities
$
25,493

 
$

 
$

 
$
25,493

Valuation of senior notes
 
March 31, 2019
 
December 31, 2018
 
Carrying Amount
 
Estimated Fair Value
 
Carrying Amount
 
Estimated Fair Value
Senior Notes
$
3,130,921

 
$
3,209,279

 
$
3,130,122

 
$
3,116,046