Amount of cash and cash equivalents restricted as to withdrawal or usage, classified as current. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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

Basis of Presentation and Summary of Significant Accounting Policies (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Jan. 01, 2016
Property, Plant and Equipment [Line Items]
Cash and cash equivalents and marketable securities	$ 32,900
Prepaid materials	4,655		$ 4,783
Restricted cash				$ 1,300
Prepaid Materials
Property, Plant and Equipment [Line Items]
Prepaid materials	$ 4,700		$ 4,800
AOCI Including Portion Attributable to Noncontrolling Interest \| Accounting Standards Update 2014-09
Property, Plant and Equipment [Line Items]
Cumulative effect of new accounting principle in period of adoption		$ (1,800)