Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	FIDELITY SECURITIES FUND
Prospectus Date	rr_ProspectusDate	Sep. 29, 2017
Retail | Fidelity Flex Large Cap Growth Fund
Risk/Return:	rr_RiskReturnAbstract
Supplement [Text Block]	fsf_SupplementTextBlock	Supplement to the Fidelity Flex℠ Funds Fidelity Flex℠ Large Cap Growth Fund September 29, 2017 Prospectus The following information replaces footnote b to the "Annual Operating Expenses" table found in the "Fund Summary" section. bThe fund is available only to certain fee-based accounts offered by Fidelity. Managed account clients, retirement plans, plan sponsors and/or plan participants pay a wrap fee that covers investment advisory and administrative services.