Quarterly report pursuant to Section 13 or 15(d)

SELF STORAGE PROPERTY ACQUISITIONS - Narrative (Details)

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SELF STORAGE PROPERTY ACQUISITIONS - Narrative (Details) (USD $)
0 Months Ended 1 Months Ended 3 Months Ended 2 Months Ended
Jan. 01, 2015
property
Jan. 31, 2015
property
Mar. 31, 2015
property
Mar. 31, 2014
May 31, 2015
property
Business Acquisition [Line Items]          
Number of self storage properties acquired 5us-gaap_NumberOfBusinessesAcquired 6us-gaap_NumberOfBusinessesAcquired 6us-gaap_NumberOfBusinessesAcquired    
Estimated fair value of acquired self storage properties     $ 41,000,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet    
Pro forma revenue     1,200,000us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual    
Pro forma operating income     700,000us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual    
Acquisition costs     599,000us-gaap_BusinessCombinationAcquisitionRelatedCosts 182,000us-gaap_BusinessCombinationAcquisitionRelatedCosts  
Number of properties under contract to acquire     21nsa_NumberOfBusinessesToBeAcquired    
Leases, Acquired-in-Place [Member]          
Business Acquisition [Line Items]          
Recognized fair value of identifiable intangible assets     $ 1,200,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_LeasesAcquiredInPlaceMember
   
Property Subject to Operating Lease [Member]          
Business Acquisition [Line Items]          
Number of self storage properties acquired     3us-gaap_NumberOfBusinessesAcquired
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= us-gaap_PropertySubjectToOperatingLeaseMember
   
Subsequent Event [Member]          
Business Acquisition [Line Items]          
Number of self storage properties acquired         21us-gaap_NumberOfBusinessesAcquired
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember