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

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December 31, 2019

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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

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​

​

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(Level 1)

    

(Level 2)

    

(Level 3)

    

Total

Marketable securities

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$

32,201

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$

—

​

$

—

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$

32,201

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​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

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

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$

25,493

​

$

—

​

$

—

​

$

25,493

Valuation of senior notes

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December 31, 2019

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December 31, 2018

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Carrying Amount

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Estimated Fair Value

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Carrying Amount

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Estimated Fair Value

Senior Notes

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$

3,629,527

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$

3,881,925

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$

3,130,122

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$

3,116,046