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Fair Value Measurements (Tables)
3 Months Ended
Mar. 31, 2020
Fair Value Measurements  
Valuation of marketable securities

March 31, 2020

Quoted Priced in Active Markets

Significant Other

Significant

for Identical Instruments

Observable Inputs

Unobservable Inputs

    

(Level 1)

    

(Level 2)

    

(Level 3)

    

Total

Marketable securities

$

28,141

$

$

$

28,141

December 31, 2019

Quoted Prices 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

Valuation of senior notes

March 31, 2020

December 31, 2019

Carrying Amount

Estimated Fair Value

Carrying Amount

Estimated Fair Value

Senior Notes

$

4,127,248

$

4,220,514

$

3,629,527

$

3,881,925