Investments - Additional Information (Detail) $ in Thousands	Dec. 31, 2017 USD ($) Securities	Dec. 31, 2016 USD ($) Securities
Investments, Debt and Equity Securities [Abstract]
Number of debt securities with unrealized loss position for less than one year	54	58
Aggregate fair value of debt securities \| $	$ 96,623	$ 95,949
Number of debt securities with unrealized loss position for greater than one year	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