Fair value of financial instruments and investments - Marketable Securities, Unrealized Gains (Losses) (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Transfers of assets between Level 1 and Level 2 of the fair value measurement hierarchy	$ 0
Marketable securities accounted for as available-for-sale securities
Amortized Cost	79,432,000	$ 173,347,000
Gross Unrealized Gains	52,000	101,000
Gross Unrealized Losses	(30,000)	(103,000)
Fair Value	79,454,000	173,345,000
Commercial paper
Marketable securities accounted for as available-for-sale securities
Amortized Cost	13,775,000	12,919,000
Gross Unrealized Gains	52,000	47,000
Gross Unrealized Losses	0	0
Fair Value	13,827,000	12,966,000
Corporate debt securities
Marketable securities accounted for as available-for-sale securities
Amortized Cost	65,657,000	153,240,000
Gross Unrealized Gains	0	52,000
Gross Unrealized Losses	(30,000)	(103,000)
Fair Value	65,627,000	153,189,000
Government obligations
Marketable securities accounted for as available-for-sale securities
Amortized Cost	0	7,188,000
Gross Unrealized Gains	0	2,000
Gross Unrealized Losses	0	0
Fair Value	$ 0	$ 7,190,000

Amount of transfers of assets measured on a recurring basis between Level 1 and Level 2 of the fair value hierarchy.

No definition available.

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- References

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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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