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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Fair Value Measurements    
Marketable securities $ 473,362 $ 503,503 [1]
Fair Value of Financial Instruments    
Carrying amount of long-term debt 2,969,380 2,687,024
Recurring    
Fair Value Measurements    
Marketable securities 473,362 503,503
Cash, Cash Equivalents and Restricted Cash 175,728 181,792
Total Assets Measured at Fair Value 649,090 685,295
Cost of cash and cash equivalents and available for sale securities 649,100 685,500
Fair value of cash and cash equivalents and available for sale securities 649,100 685,300
Recurring | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 206,184 344,251
Recurring | Commercial paper    
Fair Value Measurements    
Marketable securities 267,178 159,252
Recurring | Level 1    
Fair Value Measurements    
Cash, Cash Equivalents and Restricted Cash 175,728 181,792
Total Assets Measured at Fair Value 175,728 181,792
Recurring | Level 2    
Fair Value Measurements    
Marketable securities 473,362 503,503
Total Assets Measured at Fair Value 473,362 503,503
Fair Value of Financial Instruments    
Fair value of long-term debt 3,000,000 2,700,000
Carrying amount of long-term debt 2,990,000 2,710,000
Recurring | Level 2 | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 206,184 344,251
Recurring | Level 2 | Commercial paper    
Fair Value Measurements    
Marketable securities $ 267,178 $ 159,252
[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.