Restricted Cash and Short-term Investments - Tabular Disclosure (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Short-term investments
Short-term investments, amortized cost	$ 930,555	$ 598,308
Short-term investments, gross unrealized gains	653	12
Short-term investments, gross unrealized losses		406
Short-term investments	931,208	597,914
U.S. Treasury Securities
Short-term investments
Available-for-sale securities, amortized cost	902,771	561,733
Available-for-sale securities, gross unrealized gains	644
Available-for-sale securities. gross unrealized losses		406
Available-for-sale securities	903,415	561,327
U.S. Agency Securities
Short-term investments
Available-for-sale securities, amortized cost	17,612	17,651
Available-for-sale securities, gross unrealized gains	9	12
Available-for-sale securities	17,621	17,663
Time Deposits
Short-term investments
Time deposits, amortized cost	10,172	18,924
Time deposits	$ 10,172	$ 18,924

Short-term investments, accumulated gross unrealized gain, before tax.

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Short-term investments, accumulated gross unrealized loss, before tax.

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Short-term investments, amortized cost basis.

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Time deposits, amortized cost basis.

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Time deposits, fair value disclosure.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount of investments in debt securities classified as available-for-sale, expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer.

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Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

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