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Fair Value Measurements - Schedule of financial instruments that are not carried at fair value (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Assets: |  |  |
Carrying Amount, Short-term investments | $ 0 | [1] | $ 21,923
Carrying Amount, Long-term Investments | 0 | [1] | 69,699
Assets: |  |  |
Fair Value, Short-term investments | 0 | [1] | 21,858
Fair Value, Long-term investments | 0 | [1] | 69,090
2023 Notes [Member] |  |  |
Liabilities: |  |  |
Notes Payable | 750,000 |  | 750,000
Liabilities: |  |  |
Notes Payable Fair Value Disclosure | 759,375 |  | 736,725
2027 Notes [Member] |  |  |
Liabilities: |  |  |
Notes Payable | 1,000,000 |  | 500,000
Liabilities: |  |  |
Notes Payable Fair Value Disclosure | $ 1,010,000 |  | $ 482,500
[1] | Investments were measured at fair value as of March 31, 2019. See table above.

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