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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Fair Value Measurements    
Marketable Securities $ 263,597 $ 360,945
Fair Value of Financial Instruments    
Carrying amount of long-term debt 2,984,818 3,159,974
Recurring    
Fair Value Measurements    
Cost of cash and cash equivalents and available for sale securities 571,400 689,400
Recurring | Fair value    
Fair Value Measurements    
Cash and cash equivalents 307,909 328,384
Marketable Securities 263,597 360,945
Total assets measured at fair value 571,506 689,329
Recurring | Fair value | Bonds and bond funds    
Fair Value Measurements    
Marketable Securities 207,538 229,783
Recurring | Fair value | Commercial paper    
Fair Value Measurements    
Marketable Securities 56,059 131,162
Recurring | Level 1    
Fair Value Measurements    
Cash and cash equivalents 307,909 328,384
Total assets measured at fair value 307,909 328,384
Recurring | Level 2    
Fair Value Measurements    
Marketable Securities 263,597 360,945
Total assets measured at fair value 263,597 360,945
Fair Value of Financial Instruments    
Fair value of long-term debt 2,980,000 3,160,000
Carrying amount of long-term debt 3,010,000 3,190,000
Recurring | Level 2 | Bonds and bond funds    
Fair Value Measurements    
Marketable Securities 207,538 229,783
Recurring | Level 2 | Commercial paper    
Fair Value Measurements    
Marketable Securities $ 56,059 $ 131,162