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Cash, cash equivalents and short-term investments - Summary (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 147,644	$ 126,832
Gross Unrealized Gains	19	0
Gross Unrealized Losses	(10)	(170)
Fair Value	147,653	126,662
Cash and money market funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	95,894	43,224
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	95,894	43,224
Corporate debt securities due in one year or less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	50,549	68,898
Gross Unrealized Gains	17	0
Gross Unrealized Losses	(10)	(107)
Fair Value	50,556	68,791
Corporate debt securities, due in two years or less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,201	14,710
Gross Unrealized Gains	2	0
Gross Unrealized Losses	0	(63)
Fair Value	1,203	14,647
Cash and cash equivalents
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	95,894	43,224
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	95,894	43,224
Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	51,750	83,608
Gross Unrealized Gains	19	0
Gross Unrealized Losses	(10)	(170)
Fair Value	$ 51,759	$ 83,438

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