Cash, cash equivalents, short-term investments and restricted cash - Narrative (Details)	12 Months Ended
Dec. 31, 2019 USD ($) investment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Cash and Cash Equivalents [Abstract]
Impairment losses on its marketable securities \| $	$ 0	$ 0	$ 0
Investments in marketable securities in unrealized loss position in excess of 12 months item \| investment	0

Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without an allowance for credit loss. Includes beneficial interest in securitized financial asset.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922888-210455

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant