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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Fair Value Measurements    
Marketable securities $ 360,945 $ 503,503
Total Assets Measured at Fair Value 689,372 685,523
Fair Value of Financial Instruments    
Fair value of long-term debt 3,157,300 2,698,400
Recurring | Fair value    
Fair Value Measurements    
Marketable securities 360,945 503,503
Cash, Cash Equivalents and Restricted Cash 328,384 181,792
Total Assets Measured at Fair Value 689,329 685,295
Recurring | Fair value | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 229,783 344,251
Recurring | Fair value | Commercial paper    
Fair Value Measurements    
Marketable securities 131,162 159,252
Recurring | Level 1    
Fair Value Measurements    
Cash, Cash Equivalents and Restricted Cash 328,384 181,792
Total Assets Measured at Fair Value 328,384 181,792
Recurring | Level 2    
Fair Value Measurements    
Marketable securities 360,945 503,503
Total Assets Measured at Fair Value 360,945 503,503
Recurring | Level 2 | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 229,783 344,251
Recurring | Level 2 | Commercial paper    
Fair Value Measurements    
Marketable securities $ 131,162 $ 159,252