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Fair value of financial instruments Schedule of assets and liabilities measured at fair value on a recurring basis, by level (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2013
Dec. 31, 2012
Assets, Fair Value Disclosure [Abstract]    
Cash and Cash Equivalents, Fair Value Disclosure $ 39,652 $ 92,521
Available-for-sale Securities 88,024 34,989
Fair Value, Measurements, Recurring | Level 1
    
Assets, Fair Value Disclosure [Abstract]    
Total assets measured at fair value 22,991 45,040
Fair Value, Measurements, Recurring | Level 2
    
Assets, Fair Value Disclosure [Abstract]    
Total assets measured at fair value 89,234 42,488
Fair Value, Measurements, Recurring | Total
    
Assets, Fair Value Disclosure [Abstract]    
Total assets measured at fair value 112,225 87,528
Fair Value, Measurements, Recurring | Money Market Funds [Member] | Level 1
    
Assets, Fair Value Disclosure [Abstract]    
Cash and Cash Equivalents, Fair Value Disclosure 22,991 45,040
Fair Value, Measurements, Recurring | Money Market Funds [Member] | Level 2
    
Assets, Fair Value Disclosure [Abstract]    
Cash and Cash Equivalents, Fair Value Disclosure 0 0
Fair Value, Measurements, Recurring | Money Market Funds [Member] | Total
    
Assets, Fair Value Disclosure [Abstract]    
Cash and Cash Equivalents, Fair Value Disclosure 22,991 45,040
Fair Value, Measurements, Recurring | Commercial paper | Level 1
    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities 0 0
Fair Value, Measurements, Recurring | Commercial paper | Level 2
    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities 900 32,487
Fair Value, Measurements, Recurring | Commercial paper | Total
    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities 900 32,487
Fair Value, Measurements, Recurring | U.S. government agency securities | Level 1
    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities 0 0
Fair Value, Measurements, Recurring | U.S. government agency securities | Level 2
    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities 11,279 10,001
Fair Value, Measurements, Recurring | U.S. government agency securities | Total
    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities 11,279 10,001
Fair Value, Measurements, Recurring | US Treasury Securities [Member] | Level 1
    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities 0 0
Fair Value, Measurements, Recurring | US Treasury Securities [Member] | Level 2
    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities 5,235 0
Fair Value, Measurements, Recurring | US Treasury Securities [Member] | Total
    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities 5,235 0
Fair Value, Measurements, Recurring | US States and Political Subdivisions Debt Securities [Member] | Level 1
    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities 0 0
Fair Value, Measurements, Recurring | US States and Political Subdivisions Debt Securities [Member] | Level 2
    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities 4,765 0
Fair Value, Measurements, Recurring | US States and Political Subdivisions Debt Securities [Member] | Total
    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities 4,765 0
Fair Value, Measurements, Recurring | Corporate Debt Securities [Member] | Level 1
    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities 0 0
Fair Value, Measurements, Recurring | Corporate Debt Securities [Member] | Level 2
    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities 67,055 0
Fair Value, Measurements, Recurring | Corporate Debt Securities [Member] | Total
    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities $ 67,055 $ 0