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Legal Proceedings, Commitments and Contingencies - Schedule of Contractual Obligations and Future Cash Payments (Parenthetical) (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract] |  |  |
Operating lease expenses | $ 291,000 | $ 116,000 | $ 72,000
Operating lease payment due upon lease termination | $ 0 |  |

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- Definition Operating Lease Expense + References No definition available. + Details
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Namespace Prefix: | gale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Operating Leases Future Minimum Payments Due Upon Termination of Leases + References No definition available. + Details
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Namespace Prefix: | gale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration