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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Fair Value Measurements    
Marketable securities $ 432,966 $ 360,945
Total assets measured at fair value 520,172 689,372
Fair Value of Financial Instruments    
Fair value of long-term debt 3,049,100 3,157,300
Recurring | Fair value    
Fair Value Measurements    
Marketable securities 432,966 360,945
Cash, Cash Equivalents and Restricted Cash 87,206 328,384
Total assets measured at fair value 520,172 689,329
Recurring | Fair value | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 267,243 229,783
Recurring | Fair value | Commercial paper    
Fair Value Measurements    
Marketable securities 165,723 131,162
Recurring | Level 1    
Fair Value Measurements    
Cash, Cash Equivalents and Restricted Cash 87,206 328,384
Total assets measured at fair value 87,206 328,384
Recurring | Level 2    
Fair Value Measurements    
Marketable securities 432,966 360,945
Total assets measured at fair value 432,966 360,945
Recurring | Level 2 | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 267,243 229,783
Recurring | Level 2 | Commercial paper    
Fair Value Measurements    
Marketable securities $ 165,723 $ 131,162