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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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Fair value of long term investsments.

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The total amount of investments that are intended to be held for an extended period of time (longer than one operating cycle).

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