Investments - Additional Information (Detail) $ in Millions	Mar. 31, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Investments Debt And Equity Securities [Abstract]
Available-for-sale, investment securities held with maturity dates of more than one year and less than two years \| $	$ 35.5	$ 31.8
Number of available-for-sale investment securities in unrealized loss position \| Security	24	57

Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-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_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant