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Cash, cash equivalents and short-term investments (Details) (USD $) | 3 Months Ended | 6 Months Ended |  |  |  |  |  |  | 3 Months Ended | 6 Months Ended | 18 Months Ended |
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Jun. 30, 2013 item | Jun. 30, 2012 | Jun. 30, 2013 item | Jun. 30, 2012 | Dec. 31, 2012 | Dec. 31, 2011 | Jun. 30, 2013 Corporate debt securities due in one year or less | Dec. 31, 2012 Corporate debt securities due in one year or less | Jun. 30, 2013 Corporate debt securities due in two years or less | Dec. 31, 2012 Corporate debt securities due in two years or less | Dec. 31, 2012 Marketable securities | Jun. 30, 2013 Short-term investments | Jun. 30, 2012 Short-term investments | Jun. 30, 2013 Short-term investments | Jun. 30, 2013 Short-term investments | Jun. 30, 2013 Financial institution debt securities and other corporate debt securities located in Canada, the United Kingdom, the Netherlands, Australia, and Norway
Cash, cash equivalents and marketable securities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash and money market funds, Amortized Cost | $ 28,193,000 |  | $ 28,193,000 |  | $ 14,776,000 |  |  |  |  |  |  |  |  |  |  |
Cash and money market funds, Fair Value | 28,193,000 |  | 28,193,000 |  | 14,776,000 |  |  |  |  |  |  |  |  |  |  |
Amortized Cost | 54,371,000 |  | 54,371,000 |  | 31,397,000 |  | 20,251,000 | 5,754,000 | 5,927,000 | 10,867,000 | 16,621,000 | 26,178,000 |  | 26,178,000 | 26,178,000 | 13,300,000
Gross Unrealized Gains | 1,000 |  | 1,000 |  | 5,000 |  | 1,000 | 2,000 |  | 3,000 | 5,000 | 1,000 |  | 1,000 | 1,000 |
Gross Unrealized Losses | (27,000) |  | (27,000) |  | (11,000) |  | (16,000) | (1,000) | (11,000) | (10,000) | (11,000) | (27,000) |  | (27,000) | (27,000) |
Fair Value | 54,345,000 |  | 54,345,000 |  | 31,391,000 |  | 20,236,000 | 5,755,000 | 5,916,000 | 10,860,000 | 16,615,000 | 26,152,000 |  | 26,152,000 | 26,152,000 |
Amortized cost of cash and cash equivalents and marketable securities | 54,588,000 |  | 54,588,000 |  | 31,397,000 |  |  |  |  |  |  |  |  |  |  |
Fair value of cash and cash equivalents and marketable securities | 54,561,000 |  | 54,561,000 |  | 31,391,000 |  |  |  |  |  |  |  |  |  |  |
Amortized cost of cash and cash equivalents | 28,193,000 | 34,067,000 | 28,193,000 | 34,067,000 | 14,776,000 | 4,541,000 |  |  |  |  |  |  |  |  |  |
Fair value of cash and cash equivalents | 28,193,000 |  | 28,193,000 |  | 14,776,000 |  |  |  |  |  |  |  |  |  |  |
Number of debt securities that had been in an unrealized loss position for less than 12 months | 21 |  | 21 |  |  |  |  |  |  |  |  |  |  |  |  |
Number of debt securities that had been in an unrealized loss position for 12 months or longer |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value of debt securities that had been in an unrealized loss position for less than 12 months | 20,200,000 |  | 20,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Realized gains |  |  |  |  |  |  |  |  |  |  |  | 0 | 0 | 0 | 0 |
Realized losses |  |  |  |  |  |  |  |  |  |  |  | 0 | 0 | 0 | 0 |
Other-than-temporary impairments recognized | $ 0 | $ 0 | $ 0 | $ 0 |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash -URI http://asc.fasb.org/extlink&oid=6506951 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash Equivalents -URI http://asc.fasb.org/extlink&oid=6507016 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 8, 9 -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 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 7 -Footnote 1 -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 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 1 -Article 5 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6361293&loc=d3e6676-107765 Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3044-108585 Reference 9: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 7, 26 -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. + Details
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