Other Income (Narrative) (Details) - USD ($)	3 Months Ended					6 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Other Income [Line Items]
Cash proceeds from insurance settlement						$ 272,000	$ 213,000
Other income (expense)		$ 31,000		$ 0		$ 303,000	$ 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
Forecast | Tower in Des Moines Iowa [Member]
Other Income [Line Items]
Other income (expense)	$ 290,000