Cash Equivalents and Investments - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Cash Equivalents And Investments [Line Items]
Cash equivalents	$ 0.3	$ 9.2
Cash Equivalents [Member]
Cash Equivalents And Investments [Line Items]
Cash equivalents	$ 0.3	$ 9.2
Maximum [Member]
Cash Equivalents And Investments [Line Items]
Long-term investments maturity period, in years	6 years
Short-Term investments maturity period, in years	1 year
Minimum [Member]
Cash Equivalents And Investments [Line Items]
Long-term investments maturity period, in years	1 year

Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

No definition available.

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