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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Fair Value Measurements    
Marketable securities $ 610,185 $ 432,966
Total assets measured at fair value 825,908 520,172
Fair Value of Financial Instruments    
Fair value of long-term debt 3,244,000 3,049,100
Recurring | Fair value    
Fair Value Measurements    
Marketable securities 610,185 432,966
Cash, Cash Equivalents and Restricted Cash 215,723 87,206
Total assets measured at fair value 825,908 520,172
Recurring | Fair value | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 117,928 267,243
Recurring | Fair value | Commercial paper    
Fair Value Measurements    
Marketable securities 492,257 165,723
Recurring | Level 1    
Fair Value Measurements    
Cash, Cash Equivalents and Restricted Cash 215,723 87,206
Total assets measured at fair value 215,723 87,206
Recurring | Level 2    
Fair Value Measurements    
Marketable securities 610,185 432,966
Total assets measured at fair value 610,185 432,966
Recurring | Level 2 | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 117,928 267,243
Recurring | Level 2 | Commercial paper    
Fair Value Measurements    
Marketable securities $ 492,257 $ 165,723