National Storage Affiliates Trust Quarterly report pursuant to Section 13 or 15(d)

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands	Sep. 30, 2021 USD ($) property partnership	Dec. 31, 2020 USD ($) partnership property
Variable Interest Entity [Line Items]
Number of self storage properties \| property	940
Net book value of real estate owned	$ 4,133,049	$ 3,195,569
Carrying value of fixed rate mortgages	2,135,005	1,922,614
Fixed rate mortgages payable \| Fixed rate mortgages payable
Variable Interest Entity [Line Items]
Carrying value of fixed rate mortgages	$ 305,005	$ 223,614
VIE, Primary Beneficiary
Variable Interest Entity [Line Items]
Number of partnerships considered to be VIEs \| partnership	22	21
Number of self storage properties \| property	48	34
Net book value of real estate owned	$ 429,700	$ 225,100
VIE, Primary Beneficiary \| Fixed rate mortgages payable \| Fixed rate mortgages payable
Variable Interest Entity [Line Items]
Carrying value of fixed rate mortgages	$ 188,700	$ 100,700