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Fair Value Measurements - Schedule of financial instruments that are not carried at fair value (Details) - USD ($) $ in Thousands | Jun. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Assets: |  |
Carrying Amount, Short-term investments |  | $ 21,923
Carrying Amount, Long-term Investments |  | 69,699
Assets: |  |
Fair Value, Short-term investments |  | 21,858
Fair Value, Long-term investments |  | 69,090
2023 Notes [Member] |  |
Liabilities: |  |
Notes Payable | $ 750,000 | 750,000
Liabilities: |  |
Notes Payable Fair Value Disclosure | 738,975 | 736,725
2027 Notes [Member] |  |
Liabilities: |  |
Notes Payable | 1,000,000 | 500,000
Liabilities: |  |
Notes Payable Fair Value Disclosure | $ 983,700 | $ 482,500

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