Note 5 - Common Stock Investment (Details Textual) - $ / shares	Nov. 06, 2020	Sep. 30, 2021	Dec. 31, 2020	Mar. 09, 2020	Jan. 31, 2020
Common Stock, Par or Stated Value Per Share (in dollars per share)		$ 0.01	$ 0.01	$ 0.01	$ 0.01
Conversion of JCAP Preferred Stock into NSP Common Stock [Member]
Conversion of Stock, Shares Issued Per Each Share Converted (in shares)	1
Conversion of Stock, Price Per Share Converted (in dollars per share)	$ 1,063.47
NexPoint Storage Partners [Member]
Equity Method Investment, Ownership Percentage		25.80%
NexPoint Storage Partners [Member]
Common Stock, Par or Stated Value Per Share (in dollars per share)	$ 0.01
Common Stock, Accrued Interest and Dividend Rate, Percentage	5.00%
Jernigan Capital [Member]
Share Price (in dollars per share)	$ 17.30