Short-term investment - Additional Information (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Maturities of Time Deposits, Description	the time deposits with original maturities between three months and one year
Time deposits, Interest income	$ 2.4
Minimum
Schedule Of Available For Sale Securities [Line Items]
Time deposits, Maturity period	3 months
Maximum
Schedule Of Available For Sale Securities [Line Items]
Time deposits, Maturity period	1 year

Interest income derived from funds deposited with both domestic and foreign financial institutions including funds in money market and other accounts.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.4)
-URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260

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Description of time deposit maturities, including certificates of deposit.

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Period of time between issuance and maturity of customer deposits, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher SEC
-Name Industry Guide
-Number 3
-Section V
-Paragraph D-E

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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