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Commitments and Contingencies - Additional Information (Details) (USD $)
In Thousands, unless otherwise specified
0 Months Ended 1 Months Ended 12 Months Ended
May 01, 2014
Feb. 28, 2014
Jul. 31, 2011
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Disclosure Commitments And Contingencies Additional Information Details [Line Items]            
Tenant improvement allowance   $ 369us-gaap_PaymentsForTenantImprovements $ 376us-gaap_PaymentsForTenantImprovements      
Lease term   5 years        
Rent expense       572us-gaap_OperatingLeasesRentExpenseNet 285us-gaap_OperatingLeasesRentExpenseNet 265us-gaap_OperatingLeasesRentExpenseNet
Reduction in letter of credit on successful payment of lease amount 119rare_DiscountOnLetterOfCreditOnSuccessfulPaymentOfLeaseAmount          
Restricted cash in connection with lease agreement       $ 594us-gaap_RestrictedCashAndCashEquivalents    
Minimum            
Disclosure Commitments And Contingencies Additional Information Details [Line Items]            
Increase in percentage of monthly lease payments     3.00%rare_IncreaseInPercentageOfMonthlyLeasePayments
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
     
Maximum            
Disclosure Commitments And Contingencies Additional Information Details [Line Items]            
Increase in percentage of monthly lease payments     6.00%rare_IncreaseInPercentageOfMonthlyLeasePayments
/ us-gaap_RangeAxis
= us-gaap_MaximumMember