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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Fair Value Measurements    
Marketable Securities $ 295,900 $ 360,945
Fair Value of Financial Instruments    
Carrying amount of long-term debt 3,064,294 3,159,974
Recurring    
Fair Value Measurements    
Cost of cash and cash equivalents and available for sale securities 549,400 689,400
Recurring | Fair value    
Fair Value Measurements    
Cash and cash equivalents 253,661 328,384
Marketable Securities 295,900 360,945
Total assets measured at fair value 549,561 689,329
Recurring | Fair value | Bonds and bond funds    
Fair Value Measurements    
Marketable Securities 200,281 229,783
Recurring | Fair value | Commercial paper    
Fair Value Measurements    
Marketable Securities 95,619 131,162
Recurring | Level 1    
Fair Value Measurements    
Cash and cash equivalents 253,661 328,384
Total assets measured at fair value 253,661 328,384
Recurring | Level 2    
Fair Value Measurements    
Marketable Securities 295,900 360,945
Total assets measured at fair value 295,900 360,945
Fair Value of Financial Instruments    
Fair value of long-term debt 3,060,000 3,160,000
Carrying amount of long-term debt 3,090,000 3,190,000
Recurring | Level 2 | Bonds and bond funds    
Fair Value Measurements    
Marketable Securities 200,281 229,783
Recurring | Level 2 | Commercial paper    
Fair Value Measurements    
Marketable Securities $ 95,619 $ 131,162