Cash, cash equivalents, short-term investments and restricted cash - Summary of unrealized gains and losses (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 17,095	$ 42,843
Gross Unrealized Gains	0	4
Gross Unrealized Loss	(2)	(23)
Fair Value	17,093	42,824
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,530	199
Gross Unrealized Gains	0	0
Gross Unrealized Loss	0	0
Fair Value	11,530	199
Money market accounts
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,812	23,809
Gross Unrealized Gains	0	0
Gross Unrealized Loss	0	0
Fair Value	4,812	23,809
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	9,438
Gross Unrealized Gains	0	4
Gross Unrealized Loss	0	(3)
Fair Value	0	9,439
Government agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	3,767
Gross Unrealized Gains	0	0
Gross Unrealized Loss	0	(10)
Fair Value	0	3,757
U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	753	5,008
Gross Unrealized Gains	0	0
Gross Unrealized Loss	(2)	(10)
Fair Value	751	4,998
Sovereign government bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	622
Gross Unrealized Gains	0	0
Gross Unrealized Loss	0	0
Fair Value	0	622
Cash equivalents
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,812	23,809
Gross Unrealized Gains	0	0
Gross Unrealized Loss	0	0
Fair Value	4,812	23,809
Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,283	19,034
Gross Unrealized Gains	0	4
Gross Unrealized Loss	(2)	(23)
Fair Value	$ 12,281	$ 19,015

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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