Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Additional Information (Detail)

v2.4.1.9
Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended 35 Months Ended
Mar. 31, 2014
Dec. 31, 2014
Dec. 31, 2014
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]      
Reportable segment   1us-gaap_NumberOfReportableSegments  
Property and equipment, estimated useful lives   3 years  
Leasehold Improvements | Minimum      
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]      
Property and equipment, estimated useful lives   4 years  
Leasehold Improvements | Maximum      
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]      
Property and equipment, estimated useful lives   6 years  
Merck KGaA      
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]      
Percentage share of eligible worldwide development expenses   70.00%thld_WorldwideDevelopmentExpensesSharePercentage
/ dei_LegalEntityAxis
= thld_MerckKGaAMember
 
Upfront and milestone payments received to date $ 12.5us-gaap_ProceedsFromCollaborators
/ dei_LegalEntityAxis
= thld_MerckKGaAMember
$ 110.0us-gaap_ProceedsFromCollaborators
/ dei_LegalEntityAxis
= thld_MerckKGaAMember
$ 110.0us-gaap_ProceedsFromCollaborators
/ dei_LegalEntityAxis
= thld_MerckKGaAMember