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Cash Equivalents and Investments (Details) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 USD ($) investment | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) investment | Sep. 30, 2017 USD ($)
Investments Debt And Equity Securities [Abstract] |  |  |  |
Realized gains (losses) recognized on the sale or maturity of available-for-sale securities | $ | $ 0 | $ 0 | $ 0 | $ 0
Securities in continuous unrealized loss position for more than one year | investment | 0 |  | 0 |

X
- Definition This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Number of investment positions in available-for-sale investments that have been in a continuous unrealized loss position for greater than or equal to a year for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement. + References No definition available. + Details
Name: | us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_InvestmentsDebtAndEquitySecuritiesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration