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Fair Value Measurements (Tables)
9 Months Ended
Sep. 30, 2020
Fair Value Measurements  
Valuation of marketable securities

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​

​

​

​

​

​

​

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September 30, 2020

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​

Quoted Priced in Active Markets

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Significant Other

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Significant

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​

​

​

​

for Identical Instruments

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Observable Inputs

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Unobservable Inputs

​

​

​

​

    

(Level 1)

    

(Level 2)

    

(Level 3)

    

Total

Marketable securities

​

$

35,627

​

$

—

​

$

—

​

$

35,627

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

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

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September 30, 2020

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December 31, 2019

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Carrying Amount

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Estimated Fair Value

​

Carrying Amount

​

Estimated Fair Value

Senior Notes

​

$

4,622,207

​

$

5,135,416

​

$

3,629,527

​

$

3,881,925