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Investments - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]      
SERP investments $ 840   $ 0
SERP Investments at fair value 2,039   2,687
CoBank      
Schedule of Equity Method Investments [Line Items]      
SERP Investments at fair value 10,000   $ 9,800
Other nonoperating income (expense) $ 1,000 $ 1,000  
Percentage of patronage credit paid in cash 75.00%    
Percentage of patronage credit paid in share 25.00%    
Valley Network Partnership      
Schedule of Equity Method Investments [Line Items]      
Ownership interest, percentage 20.00%    
Equity Method Investee      
Schedule of Equity Method Investments [Line Items]      
Revenue from related parties $ 200 200  
Related party costs $ 500 $ 800