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Long-Term Debt (Details) - USD ($)
12 Months Ended
Feb. 27, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Long-term Debt [Abstract]        
Payments on long-term debt   $ 150,000,000 $ 125,000,000 $ 0
Original issue discount   6,454,000 5,249,000 [1]  
Senior Notes Due March 1, 2028 [Member]        
Long-term Debt [Abstract]        
Long term debt, face amount   $ 500,000,000    
Debt instrument, interest rate, stated percentage   4.20%    
Maturity Date   Mar. 01, 2028    
Periodic payment, interest   $ 10,500,000    
Cash proceeds received   495,019,000    
Original issue discount   705,000    
Debt issuance costs   $ 4,276,000    
Senior Notes Series A Due February 27, 2020 [Member]        
Long-term Debt [Abstract]        
Payments on long-term debt $ 150,000,000      
Debt instrument, interest rate, stated percentage   2.72%    
Maturity Date   Feb. 27, 2020    
Senior Notes Series B Due February 27, 2023 [Member]        
Long-term Debt [Abstract]        
Long term debt, face amount   $ 350,000,000    
Debt instrument, interest rate, stated percentage   3.29%    
Maturity Date   Feb. 27, 2023    
Line Of Credit [Member]        
Long-term Debt [Abstract]        
Maximum borrowing capacity   $ 10,000,000    
Credit facility, expiration date   Jun. 30, 2021    
Credit facility, amount outstanding   $ 1,007,000    
Secured Debt [Member]        
Long-term Debt [Abstract]        
Short-term debt   $ 40,000,000 $ 16,000,000  
[1] Excludes $2,650,000 attributable to the Revolving Credit Facility included in other assets at December 31, 2019.