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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Fair Value Measurements    
Marketable securities $ 482,764 $ 409,898
Fair Value of Financial Instruments    
Carrying amount of long-term debt 2,614,915 2,545,511
Recurring    
Fair Value Measurements    
Cash, Cash Equivalents and Restricted Cash 103,691 155,009
Total Assets Measured at Fair Value 586,455 564,907
Cost of cash and cash equivalents and available for sale securities 578,300 556,800
Fair value of cash and cash equivalents and available for sale securities 578,200 556,700
Recurring | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 420,812 409,885
Recurring | Asset-backed securities    
Fair Value Measurements    
Marketable securities 482,764 409,898
Recurring | Commercial paper    
Fair Value Measurements    
Marketable securities 61,952 13
Recurring | Level 1    
Fair Value Measurements    
Cash, Cash Equivalents and Restricted Cash 103,691 155,009
Total Assets Measured at Fair Value 103,691 155,009
Recurring | Level 2    
Fair Value Measurements    
Total Assets Measured at Fair Value 482,764 409,898
Fair Value of Financial Instruments    
Fair value of long-term debt 2,640,000 2,570,000
Carrying amount of long-term debt 2,640,000 2,570,000
Recurring | Level 2 | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 420,812 409,885
Recurring | Level 2 | Asset-backed securities    
Fair Value Measurements    
Marketable securities 482,764 409,898
Recurring | Level 2 | Commercial paper    
Fair Value Measurements    
Marketable securities $ 61,952 $ 13