Cash, Cash Equivalents and Short-term Investments Classification by contractual maturity of cash, cash equivalents and short-term investments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Fair value	$ 25,704	$ 34,276
Fair value	204,164	186,894
Cash, cash equivalents and short-term investments	229,868	221,170
Maturity up to one year [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	25,704	34,276
Fair value	113,010	109,451
Cash, cash equivalents and short-term investments	138,714	143,727
maturity between 1 and 2 years [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	0	0
Fair value	91,154	77,443
Cash, cash equivalents and short-term investments	$ 91,154	$ 77,443