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Senior Notes (Summary of Senior Notes) (Parenthetical) (Details) - USD ($)
$ in Thousands
1 Months Ended 12 Months Ended
Feb. 15, 2030
Oct. 27, 2017
Jul. 31, 2016
Dec. 31, 2020
May 31, 2020
Dec. 31, 2019
Oct. 31, 2017
Dec. 31, 2015
Jul. 31, 2014
Debt Instrument [Line Items]                  
Long-term Debt       $ 1,112,409   $ 1,017,010      
Senior notes 4.250% due 2024 [Member]                  
Debt Instrument [Line Items]                  
Stated interest rate       4.25%         4.25%
Long-term Debt                 $ 300,000
Debt instrument, maturity date       Jul. 31, 2024          
Redemption price, percentage of principal amount       100.00%          
Additional issuance of long-term debt     $ 200,000            
Senior notes 5.20% due 2047 [Member]                  
Debt Instrument [Line Items]                  
Stated interest rate       5.20%     5.20%    
Long-term Debt             $ 200,000    
Debt instrument, maturity date       Oct. 31, 2047          
Redemption price, percentage of principal amount       100.00%          
Additional issuance of long-term debt   $ 25,000              
Senior notes 4.00% due 2030 [Member]                  
Debt Instrument [Line Items]                  
Stated interest rate       4.00% 4.00%        
Long-term Debt         $ 400,000        
Debt instrument, maturity date       May 31, 2030          
Senior notes 4.00% due 2030 [Member] | After February 15, 2030 [Member]                  
Debt Instrument [Line Items]                  
Redemption price, percentage of principal amount       100.00%          
Senior notes 4.00% due 2030 [Member] | Measurement Input, Discount Rate [Member] | Prior to February 15, 2030 [Member]                  
Debt Instrument [Line Items]                  
Redemption at treasury rate basis point       50.00%          
Senior notes 4.00% due 2030 [Member] | Forecast                  
Debt Instrument [Line Items]                  
Redemption price, percentage of principal amount 100.00%                
Senior notes 3.50% due 2020 [Member]                  
Debt Instrument [Line Items]                  
Stated interest rate       3.50%       3.50%  
Long-term Debt               $ 300,000  
Debt instrument, maturity date       Dec. 31, 2020          
Redemption price, percentage of principal amount       100.00%