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Other Income (Narrative) (Details) - USD ($) | 3 Months Ended | 6 Months Ended | 9 Months Ended
--- | --- | --- | ---
Sep. 30, 2021 | Mar. 31, 2021 | Sep. 30, 2020 | Mar. 31, 2020 | Sep. 30, 2021 | Sep. 30, 2021 | Sep. 30, 2020
Other Income [Line Items] |  |  |  |  |  |  |
Proceeds from Insurance Settlement, Investing Activities |  |  |  |  |  | $ 567,000 | $ 213,000
Other Nonoperating Income (Expense) | $ 279,000 |  | $ 0 |  |  | $ 582,000 | $ 213,000
Tower in Keene New Hampshire [Member] |  |  |  |  |  |  |
Other Income [Line Items] |  |  |  |  |  |  |
Proceeds from Insurance Settlement, Investing Activities |  |  |  | $ 208,000 |  |  |
Other Nonoperating Income (Expense) |  |  |  | 208,000 |  |  |
Tower in Des Moines Iowa [Member] |  |  |  |  |  |  |
Other Income [Line Items] |  |  |  |  |  |  |
Proceeds from Insurance Settlement, Investing Activities | 290,000 | $ 250,000 |  |  |  |  |
Other Nonoperating Income (Expense) | $ 290,000 | $ 250,000 |  |  | $ 540,000 |  |
Tower Sites In Bellingham Washington [Member] |  |  |  |  |  |  |
Other Income [Line Items] |  |  |  |  |  |  |
Proceeds from Sale of Property, Plant, and Equipment |  |  |  | 1,700,000 |  |  |
Other Nonoperating Income (Expense) |  |  |  | $ 1,400,000 |  |  |

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