Operations and summary of significant accounting policies - Concentration of Credit Risk (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Cash and cash equivalents	$ 194,616	$ 259,386
Short-term Investments	238,000	$ 271,000
Cash, Cash Equivalents and Short-term Investments	433,000
Geographic Concentration Risk \| Cash and Cash Equivalents
Concentration Risk [Line Items]
Cash and cash equivalents	194,600
Geographic Concentration Risk \| Cash and Cash Equivalents \| Domestic
Concentration Risk [Line Items]
Cash and cash equivalents	$ 55,900
Concentration risk percentage	29.00%
Geographic Concentration Risk \| Cash and Cash Equivalents \| International
Concentration Risk [Line Items]
Cash and cash equivalents	$ 138,700
Concentration risk percentage	71.00%
Geographic Concentration Risk \| Short-term Investments
Concentration Risk [Line Items]
Short-term Investments	$ 238,000
Geographic Concentration Risk \| Short-term Investments \| Domestic
Concentration Risk [Line Items]
Short-term Investments	$ 164,300
Concentration risk percentage	69.00%
Geographic Concentration Risk \| Short-term Investments \| International
Concentration Risk [Line Items]
Short-term Investments	$ 73,700
Concentration risk percentage	31.00%
Geographic Concentration Risk \| Cash And Cash Equivalents and Short-Term Investments
Concentration Risk [Line Items]
Cash, Cash Equivalents and Short-term Investments	$ 432,600
Geographic Concentration Risk \| Cash And Cash Equivalents and Short-Term Investments \| Domestic
Concentration Risk [Line Items]
Cash, Cash Equivalents and Short-term Investments	$ 220,200
Concentration risk percentage	51.00%
Geographic Concentration Risk \| Cash And Cash Equivalents and Short-Term Investments \| International
Concentration Risk [Line Items]
Cash, Cash Equivalents and Short-term Investments	$ 212,400
Concentration risk percentage	49.00%

Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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