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Fair Value Measurements (Summary of Asset Impairment and Decommission Costs) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Fair Value Measurements [Abstract]    
Asset impairment $ 3,156 $ 11,009
Write-off of carrying value of decommissioned towers 1,327 2,699
Other (including third party decommission costs) 420 647
Total asset impairment and decommission costs $ 4,903 $ 14,355