Marketable Securities - Additional Information (Detail)	3 Months Ended	6 Months Ended
Jun. 30, 2017 USD ($) Security	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) Security	Jun. 30, 2016 USD ($)
Amortized Cost And Fair Value Debt Securities [Abstract]
Number of securities held in unrealized loss position \| Security	0		0
Realized gains (losses) on sale of available-for-sale securities \| $	$ 0	$ 0	$ 0	$ 0

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Name:	us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant