Fair value of financial instruments Schedule of assets and liabilities measured at fair value on a recurring basis, by level (Details) (USD $)	9 Months Ended	12 Months Ended
Sep. 30, 2012	Dec. 31, 2011
Assets, Fair Value Disclosure [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	$ 47,686,000
Available-for-sale Securities, Fair Value Disclosure	73,355,000
Level 2
Liabilities, Fair Value Disclosure [Abstract]
Estimated fair value of current and long-term borrowings		8,300,000
Fair Value, Measurements, Recurring \| Level 1
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	94,864,000	0
Fair Value, Measurements, Recurring \| Level 3
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities measured at fair value	0	1,853,000
Fair Value, Measurements, Recurring \| Total
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	94,864,000	0
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities measured at fair value	0	1,853,000
Fair Value, Measurements, Recurring \| Money market funds \| Level 1
Assets, Fair Value Disclosure [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	21,509,000	0
Fair Value, Measurements, Recurring \| Money market funds \| Total
Assets, Fair Value Disclosure [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	21,509,000	0
Fair Value, Measurements, Recurring \| Certificates of deposits \| Level 1
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Fair Value Disclosure	996,000	0
Fair Value, Measurements, Recurring \| Certificates of deposits \| Total
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Fair Value Disclosure	996,000	0
Fair Value, Measurements, Recurring \| Commercial paper \| Level 1
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Fair Value Disclosure	62,362,000	0
Fair Value, Measurements, Recurring \| Commercial paper \| Total
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Fair Value Disclosure	62,362,000	0
Fair Value, Measurements, Recurring \| U.S. government and agency securities \| Level 1
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Fair Value Disclosure	9,997,000	0
Fair Value, Measurements, Recurring \| U.S. government and agency securities \| Total
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Fair Value Disclosure	9,997,000	0
Convertible Preferred Stock Warrant \| Minimum
Fair Value Assumptions and Methodology for Assets and Liabilities [Abstract]
Share Price	$ 16.00	$ 13.32
Fair Value Assumptions, Risk Free Interest Rate	0.07%	0.10%
Fair Value Assumptions, Expected Volatility Rate	45.00%	45.00%
Convertible Preferred Stock Warrant \| Maximum
Fair Value Assumptions and Methodology for Assets and Liabilities [Abstract]
Share Price	$ 23.40	$ 14.16
Fair Value Assumptions, Risk Free Interest Rate	0.66%	6.00%
Fair Value Assumptions, Expected Volatility Rate	50.00%	50.00%
Convertible Preferred Stock Warrant \| Fair Value, Measurements, Recurring \| Level 1
Liabilities, Fair Value Disclosure [Abstract]
Convertible preferred stock warrants	0
Convertible Preferred Stock Warrant \| Fair Value, Measurements, Recurring \| Level 3
Liabilities, Fair Value Disclosure [Abstract]
Convertible preferred stock warrants	0	1,853,000
Convertible Preferred Stock Warrant \| Fair Value, Measurements, Recurring \| Total
Liabilities, Fair Value Disclosure [Abstract]
Convertible preferred stock warrants	$ 0	$ 1,853,000

This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized 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 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7534914&loc=d3e22054-111558

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

-URI http://asc.fasb.org/extlink&oid=6505594

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Company may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 8

-Footnote 2

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 7, 8, 9, 10

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash

-URI http://asc.fasb.org/extlink&oid=6506951

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-Name Accounting Standards Codification

-Glossary Cash Equivalents

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

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-Paragraph 2

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Risk-free interest rate assumption used in valuing an instrument.

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This element represents the aggregate of the liabilities reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

+ References

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

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Price of a single share of a number of saleable stocks of a company.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

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Convertible Preferred Stock Warrants Liability, Fair Value Disclosure

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No definition available.

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