Marketable securities - Additional Information (Detail) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, noncurrent	$ 0	$ 0

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as noncurrent.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26626-111562

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