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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Fair Value Measurements    
Marketable securities $ 482,809 $ 409,898
Fair Value of Financial Instruments    
Carrying amount of long-term debt 2,759,512 2,545,511
Recurring    
Fair Value Measurements    
Marketable securities 482,809 409,898
Cash, Cash Equivalents and Restricted Cash 151,710 155,009
Total Assets Measured at Fair Value 634,519 564,907
Cost of cash and cash equivalents and available for sale securities 626,300 556,800
Fair value of cash and cash equivalents and available for sale securities 626,300 556,700
Recurring | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 417,274 409,885
Recurring | Commercial paper    
Fair Value Measurements    
Marketable securities 65,535 13
Recurring | Level 1    
Fair Value Measurements    
Cash, Cash Equivalents and Restricted Cash 151,710 155,009
Total Assets Measured at Fair Value 151,710 155,009
Recurring | Level 2    
Fair Value Measurements    
Marketable securities 482,809 409,898
Total Assets Measured at Fair Value 482,809 409,898
Fair Value of Financial Instruments    
Fair value of long-term debt 2,800,000 2,570,000
Carrying amount of long-term debt 2,790,000 2,570,000
Recurring | Level 2 | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 417,274 409,885
Recurring | Level 2 | Commercial paper    
Fair Value Measurements    
Marketable securities $ 65,535 $ 13