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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Fair Value Measurements    
Marketable securities $ 605,438 $ 610,185
Fair Value of Financial Instruments    
Carrying amount of long-term debt 3,106,180 3,203,696
Recurring    
Fair Value Measurements    
Cost of cash and cash equivalents and available for sale securities 836,400 825,900
Recurring | Fair value    
Fair Value Measurements    
Marketable securities 605,438 610,185
Cash and Cash Equivalents 231,005 215,723
Total Assets Measured at Fair Value 836,443 825,908
Recurring | Fair value | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 92,025 117,928
Recurring | Fair value | Commercial paper    
Fair Value Measurements    
Marketable securities 513,413 492,257
Recurring | Level 1    
Fair Value Measurements    
Cash and Cash Equivalents 231,005 215,723
Total Assets Measured at Fair Value 231,005 215,723
Recurring | Level 2    
Fair Value Measurements    
Marketable securities 605,438 610,185
Total Assets Measured at Fair Value 605,438 610,185
Fair Value of Financial Instruments    
Fair value of long-term debt 3,100,000 3,200,000
Carrying amount of long-term debt 3,100,000 3,200,000
Recurring | Level 2 | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 92,025 117,928
Recurring | Level 2 | Commercial paper    
Fair Value Measurements    
Marketable securities $ 513,413 $ 492,257