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Fair Value Measurements (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Fair Value Measurements    
Marketable securities $ 409,898 $ 286,668
Fair Value of Financial Instruments    
Fair value of long-term debt 2,566,500 1,939,800
Carrying amount of long-term debt 2,545,511 1,927,901
Auction Rate Securities    
Changes in assets measured at fair value on a recurring basis using significant unobservable inputs (level 3)    
Balance 2,321  
Total realized and unrealized gains or (losses) included in earnings 161  
Total realized and unrealized gains or (losses) included in other comprehensive income 18  
Settlements (2,500)  
Recurring | Fair value    
Fair Value Measurements    
Marketable securities 409,898  
Cash, Cash Equivalents and Restricted Cash 155,009 211,251
Auction Rate Securities   2,321
Total Assets Measured at Fair Value 564,907 500,240
Recurring | Fair value | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 409,885 286,637
Recurring | Fair value | Asset-backed securities    
Fair Value Measurements    
Marketable securities 13 286,668
Recurring | Fair value | Commercial paper    
Fair Value Measurements    
Marketable securities   31
Recurring | Level 1    
Fair Value Measurements    
Cash, Cash Equivalents and Restricted Cash 155,009 211,251
Total Assets Measured at Fair Value 155,009 211,251
Recurring | Level 2    
Fair Value Measurements    
Marketable securities 409,898  
Total Assets Measured at Fair Value 409,898 286,668
Recurring | Level 2 | Bonds and bond funds    
Fair Value Measurements    
Marketable securities 409,885 286,637
Recurring | Level 2 | Asset-backed securities    
Fair Value Measurements    
Marketable securities $ 13 286,668
Recurring | Level 2 | Commercial paper    
Fair Value Measurements    
Marketable securities   31
Recurring | Level 3    
Fair Value Measurements    
Auction Rate Securities   2,321
Total Assets Measured at Fair Value   $ 2,321