Other Income (Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Other Income [Line Items]
Cash proceeds from insurance settlement	$ 272,000	$ 213,000
Other income (expense)	272,000	213,000
Tower in Keene New Hampshire [Member]
Other Income [Line Items]
Cash proceeds from insurance settlement		208,000
Other income (expense)		208,000
Tower in Des Moines Iowa [Member]
Other Income [Line Items]
Cash proceeds from insurance settlement	250,000
Other income (expense)	$ 250,000
Tower Sites In Bellingham Washington [Member]
Other Income [Line Items]
Proceeds from sale of land and buildings		1,700,000
Other income (expense)		$ 1,400,000