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Fair Value of Financial Instruments (Narrative) (Details) - USD ($)
shares in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2022
Sep. 30, 2021
Dec. 31, 2021
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Long-term Investments $ 442,620,000   $ 442,620,000   $ 274,311,000
Fair value of investments in rabbi trust 19,834,000   19,834,000   19,330,000
Asset impairments 698,000 $ 240,000 1,796,000 $ 3,468,000  
Other assets 63,798,000   63,798,000   56,716,000
Notes Receivable [Member]          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Note receivable $ 5,000,000   $ 5,000,000    
Note receivable, interest rate 6.00%   6.00%    
Note receivable, maturity Jul. 18, 2024   Jul. 18, 2024    
Long-term Investments $ 5,000,000   $ 5,000,000    
US Treasury Securities [Member]          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Investments 808,183,000   808,183,000   501,288,000
Investments maturing within one year 417,278,000   417,278,000   260,945,000
Fair value held-to-maturity investments 792,285,000   792,285,000   500,172,000
Corporate Debt Securities [Member]          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Fair value held-to-maturity investments 17,300,000 $ 18,000,000 17,300,000 18,000,000 18,000,000
Promissory note 18,000,000   18,000,000    
Reserve for expected credit losses $ 216,000   $ 216,000   $ 423,000
Note receivable, maturity Sep. 30, 2028   Sep. 30, 2028    
Maximum [Member] | US Treasury Securities [Member]          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Investment maturity term     25 months   24 months
Nuro [Member]          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Long-term Investments $ 15,968,000   $ 15,968,000   $ 15,968,000
Shares acquired 766   766    
Recognized gain     $ 5,968,000    
Tractor [Member]          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Shares acquired 4,325   4,325    
Ownership percentage 10.30%   10.30%    
Cash consideration $ 10,000,000   $ 10,000,000    
Equity method investment, impairment     0 $ 0  
Carrying value $ 12,325,000   $ 12,325,000   $ 9,251,000
Tractor Warrants [Member]          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Number of shares that may be called by warrants 3,772   3,772    
Carrying value $ 10,747,000   $ 10,747,000    
Cultivate Fund [Member]          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Carrying value $ 2,000,000   $ 2,000,000