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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Jun. 30, 2020
Dec. 31, 2019
Fair Value Measurements    
Marketable securities $ 309,823 $ 432,966
Fair Value of Financial Instruments    
Carrying amount of long-term debt 2,973,281 2,993,115
Recurring    
Fair Value Measurements    
Cost of cash and cash equivalents and available for sale securities 761,700 520,200
Recurring | Fair value    
Fair Value Measurements    
Cash and cash equivalents 452,247 87,206
Marketable securities 309,823 432,966
Total assets measured at fair value 762,070 520,172
Recurring | Fair value | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 137,183 267,243
Recurring | Fair value | Commercial paper    
Fair Value Measurements    
Marketable securities 172,640 165,723
Recurring | Level 1    
Fair Value Measurements    
Cash and cash equivalents 452,247 87,206
Total assets measured at fair value 452,247 87,206
Recurring | Level 2    
Fair Value Measurements    
Marketable securities 309,823 432,966
Total assets measured at fair value 309,823 432,966
Fair Value of Financial Instruments    
Fair value of long-term debt 3,100,000 3,000,000
Carrying amount of long-term debt 3,000,000 3,000,000
Recurring | Level 2 | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 137,183 267,243
Recurring | Level 2 | Commercial paper    
Fair Value Measurements    
Marketable securities $ 172,640 $ 165,723