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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Fair Value Measurements    
Marketable Securities $ 253,397 $ 360,945
Fair Value of Financial Instruments    
Carrying amount of long-term debt 3,097,114 3,159,974
Recurring    
Fair Value Measurements    
Cost of cash and cash equivalents and available for sale securities 544,200 689,400
Recurring | Fair value    
Fair Value Measurements    
Cash, Cash Equivalents 290,885 328,384
Marketable Securities 253,397 360,945
Total Assets Measured at Fair Value 544,282 689,329
Recurring | Fair value | Bonds and bond funds    
Fair Value Measurements    
Marketable Securities 154,407 229,783
Recurring | Fair value | Commercial paper    
Fair Value Measurements    
Marketable Securities 98,990 131,162
Recurring | Level 1    
Fair Value Measurements    
Cash, Cash Equivalents 290,885 328,384
Total Assets Measured at Fair Value 290,885 328,384
Recurring | Level 2    
Fair Value Measurements    
Marketable Securities 253,397 360,945
Total Assets Measured at Fair Value 253,397 360,945
Fair Value of Financial Instruments    
Fair value of long-term debt 3,100,000 3,160,000
Carrying amount of long-term debt 3,120,000 3,190,000
Recurring | Level 2 | Bonds and bond funds    
Fair Value Measurements    
Marketable Securities 154,407 229,783
Recurring | Level 2 | Commercial paper    
Fair Value Measurements    
Marketable Securities $ 98,990 $ 131,162