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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Mar. 31, 2020
Dec. 31, 2019
Fair Value Measurements    
Marketable securities $ 288,351 $ 432,966
Fair Value of Financial Instruments    
Carrying amount of long-term debt 2,902,192 2,993,115
Recurring    
Fair Value Measurements    
Cost of cash and cash equivalents and available for sale securities 577,900 520,200
Recurring | Fair value    
Fair Value Measurements    
Cash and cash equivalents 289,544 87,206
Marketable securities 288,351 432,966
Total assets measured at fair value 577,895 520,172
Recurring | Fair value | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 179,964 267,243
Recurring | Fair value | Commercial paper    
Fair Value Measurements    
Marketable securities 108,387 165,723
Recurring | Level 1    
Fair Value Measurements    
Cash and cash equivalents 289,544 87,206
Total assets measured at fair value 289,544 87,206
Recurring | Level 2    
Fair Value Measurements    
Marketable securities 288,351 432,966
Total assets measured at fair value 288,351 432,966
Fair Value of Financial Instruments    
Fair value of long-term debt 2,960,000 2,990,000
Carrying amount of long-term debt 2,930,000 3,020,000
Recurring | Level 2 | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 179,964 267,243
Recurring | Level 2 | Commercial paper    
Fair Value Measurements    
Marketable securities $ 108,387 $ 165,723