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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 |  |  |  |  |  |

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