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Fair Value Measurements (Tables)
12 Months Ended
Dec. 31, 2019
Fair Value Measurements  
Valuation of marketable securities

December 31, 2019

Quoted Priced in Active Markets

Significant Other

Significant

for Identical Instruments

Observable Inputs

Unobservable Inputs

    

(Level 1)

    

(Level 2)

    

(Level 3)

    

Total

Marketable securities

$

32,201

$

$

$

32,201

December 31, 2018

Quoted Prices in Active Markets

Significant Other

Significant

for Identical Instruments

Observable Inputs

Unobservable Inputs

    

(Level 1)

    

(Level 2)

    

(Level 3)

    

Total

Marketable securities

$

25,493

$

$

$

25,493

Valuation of senior notes

December 31, 2019

December 31, 2018

Carrying Amount

Estimated Fair Value

Carrying Amount

Estimated Fair Value

Senior Notes

$

3,629,527

$

3,881,925

$

3,130,122

$

3,116,046