National Storage Affiliates Trust Annual report pursuant to Section 13 and 15(d)

FAIR VALUE MEASUREMENTS - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gains forecasted to be included in earnings transferred from AOCI in the next twelve months	$ 5.4
Designated as Hedging Instrument [Member] \| Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate notional amount	$ 795.0	$ 595.0
Weighted average remaining term	4 years 6 months
Weighted average fixed rate (percent)	1.87%
Level 2 [Member] \| Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed rate mortgages, fair value disclosure	$ 276.5	$ 282.6
Weighted average market interest rate (percent)	4.17%	4.04%
Weighted average contractual interest rate (percent)	4.85%	4.87%
Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed rate mortgages, fair value disclosure	$ 268.1	$ 271.5