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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Fair Value Measurements    
Marketable securities $ 640,673 $ 610,185
Fair Value of Financial Instruments    
Carrying amount of long-term debt 2,964,951 3,203,696
Recurring | Fair value    
Fair Value Measurements    
Marketable securities 640,673 610,185
Cash and cash equivalents 271,831 215,723
Total Assets Measured at Fair Value 912,504 825,908
Cost of cash and cash equivalents and available for sale securities 912,500 825,900
Recurring | Fair value | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 49,129 117,928
Recurring | Fair value | Commercial paper    
Fair Value Measurements    
Marketable securities 591,544 492,257
Recurring | Level 1    
Fair Value Measurements    
Cash and cash equivalents 271,831 215,723
Total Assets Measured at Fair Value 271,831 215,723
Recurring | Level 2    
Fair Value Measurements    
Marketable securities 640,673 610,185
Total Assets Measured at Fair Value 640,673 610,185
Fair Value of Financial Instruments    
Fair value of long-term debt 3,000,000 3,200,000
Carrying amount of long-term debt 3,000,000 3,200,000
Recurring | Level 2 | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 49,129 117,928
Recurring | Level 2 | Commercial paper    
Fair Value Measurements    
Marketable securities $ 591,544 $ 492,257