Cash Equivalents and Marketable Securities - Additional Information (Details) - U.S. government and agency securities $ in Millions	Dec. 31, 2017 USD ($) Security	Dec. 31, 2016 USD ($) Security
Cash Equivalents And Marketable Securities [Line Items]
Fair value of debt securities in an unrealized loss position \| $	$ 37.5	$ 15.8
Number of securities in a significant unrealized loss position \| Security	0	0

This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant