Annual report pursuant to Section 13 and 15(d)

INCOME TAXES - Narrative (Details)

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INCOME TAXES - Narrative (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Mar. 31, 2015
Rate
Mar. 31, 2014
Mar. 31, 2013
Income Tax Disclosure [Abstract]      
Federal statutory rate (as a percent) 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate    
Increase (decrease) in valuation allowance $ (8.9)us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount $ (8.0)us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount $ 17.0us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
Total unrecognized tax benefit including interest and penalties 33.5us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued    
Accrued interest and penalties 1.1us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued    
Federal net operating loss 259.3us-gaap_OperatingLossCarryforwards    
Tax credit carryforwards 140.0us-gaap_TaxCreditCarryforwardAmount    
Federal net operating loss carryforwards attributable to excess tax deductions from stock option exercises 33.6qtm_OperatingLossCarryForwardsAttributableToExcessTaxDeductions    
Acquired net operating losses included in carryforwards 11.1qtm_OperatingLossCarryForwardsAcquired    
Acquired credits included in carryforwards $ 10.7qtm_TaxCreditCarryForwardAcquiredAmount