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Summary of Significant Accounting Policies (Carrying Values and Fair Values of Debt Instruments) (Details) - Senior Notes [Member] - USD ($)
12 Months Ended
Dec. 31, 2021
Sep. 20, 2021
Aug. 31, 2021
Dec. 31, 2020
Jan. 23, 2020
Senior Notes due 2021 [Member]          
Debt Instrument [Line Items]          
Carrying value of senior notes $ 100,000,000     $ 100,000,000  
Fair value of senior notes       $ 100,850,000  
Interest rate of senior notes       4.64%  
Senior note year due 2021        
New Senior Notes due 2021 [Member]          
Debt Instrument [Line Items]          
Carrying value of senior notes     $ 150,000,000 $ 150,000,000  
Fair value of senior notes       $ 150,695,000  
Interest rate of senior notes       2.39%  
Senior note year due 2021        
Senior Notes due 2022 [Member]          
Debt Instrument [Line Items]          
Carrying value of senior notes $ 125,000,000     $ 125,000,000  
Fair value of senior notes       $ 128,482,000  
Interest rate of senior notes       3.09%  
Senior note year due 2022        
Senior Notes due 2023 [Member]          
Debt Instrument [Line Items]          
Carrying value of senior notes $ 200,000,000     $ 200,000,000  
Fair value of senior notes       $ 206,204,000  
Interest rate of senior notes       2.75%  
Senior note year due 2023        
Senior Notes due 2024 [Member]          
Debt Instrument [Line Items]          
Carrying value of senior notes     150,000,000 $ 150,000,000  
Fair value of senior notes       $ 158,140,000  
Interest rate of senior notes       3.24%  
Senior note year due 2024        
Senior Notes due 2025 [Member]          
Debt Instrument [Line Items]          
Carrying value of senior notes $ 375,000,000     $ 375,000,000  
Fair value of senior notes       $ 403,025,000  
Interest rate of senior notes       3.41%  
Senior note year due 2025        
Senior Notes due 2026 [Member]          
Debt Instrument [Line Items]          
Carrying value of senior notes     400,000,000 $ 400,000,000  
Fair value of senior notes       $ 424,874,000  
Interest rate of senior notes       3.03%  
Senior note year due 2026        
Senior Notes due 2027 [Member]          
Debt Instrument [Line Items]          
Carrying value of senior notes     $ 250,000,000 $ 250,000,000  
Fair value of senior notes       $ 271,198,000  
Interest rate of senior notes       3.49%  
Senior note year due 2027        
Senior Notes due 2028 [Member]          
Debt Instrument [Line Items]          
Carrying value of senior notes $ 500,000,000     $ 500,000,000  
Fair value of senior notes $ 561,350,000     $ 597,050,000  
Interest rate of senior notes 4.25%     4.25%  
Senior note year due 2028        
Senior Notes due 2029 [Member]          
Debt Instrument [Line Items]          
Carrying value of senior notes $ 500,000,000     $ 500,000,000  
Fair value of senior notes $ 539,500,000     $ 570,450,000  
Interest rate of senior notes 3.50%     3.50%  
Senior note year due 2029        
Senior Notes due 2030 [Member]          
Debt Instrument [Line Items]          
Carrying value of senior notes $ 600,000,000     $ 600,000,000  
Fair value of senior notes $ 610,440,000     $ 644,520,000  
Interest rate of senior notes 2.60%     2.60% 2.60%
Senior note year due 2030        
Senior Notes due 2032 [Member]          
Debt Instrument [Line Items]          
Carrying value of senior notes $ 650,000,000        
Fair value of senior notes $ 637,065,000        
Interest rate of senior notes 2.20% 2.20%      
Senior note year due 2032        
Senior Notes due 2050 [Member]          
Debt Instrument [Line Items]          
Carrying value of senior notes $ 500,000,000     $ 500,000,000  
Fair value of senior notes $ 496,350,000     $ 540,050,000  
Interest rate of senior notes 3.05%     3.05%  
Senior note year due 2050        
Senior Notes due 2052 [Member]          
Debt Instrument [Line Items]          
Carrying value of senior notes $ 850,000,000        
Fair value of senior notes $ 828,580,000        
Interest rate of senior notes 2.95% 2.95%      
Senior note year due 2052