Cash, Cash Equivalents and Investments - Narrative (Details)	Mar. 31, 2018	Dec. 31, 2017
U.S. treasuries
Schedule Of Available For Sale Securities [Line Items]
Percent of total fair value of all securities in an unrealized loss position	89.00%	78.00%

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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