Cash Equivalents and Marketable Securities - Additional Information (Details) - U.S. government and agency securities $ in Millions	Jun. 30, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Cash Equivalents And Marketable Securities [Line Items]
Fair value of debt securities in an unrealized loss position \| $	$ 32.8	$ 37.5
Number of securities in a significant unrealized loss position \| Security	0	0

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.

No definition available.

Name:	us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant