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Short-term Investments - Available-For-Sale Securities Reported At Fair Value (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]          
Other than temporary impairment losses, investments, available-for-sale securities $ 0 $ 0 $ 0 $ 0  
Available-for-sale securities, gross realized gain (loss), excluding other than temporary impairments 0 $ 0 0 $ 0  
Level 1 Securities | Money market funds          
Schedule of Available-for-sale Securities [Line Items]          
Cash and Money Market Funds, Estimated Fair Value 275,248   275,248   $ 204,967
Level 2 Securities | Certificates of deposit          
Schedule of Available-for-sale Securities [Line Items]          
Level 2 securities, Estimated Fair Value         9,833
Cash          
Schedule of Available-for-sale Securities [Line Items]          
Cash and Money Market Funds, Estimated Fair Value 1,348,434   1,348,434   1,267,523
Fair Value, Measurements, Recurring          
Schedule of Available-for-sale Securities [Line Items]          
Level 2 securities, Gross Unrealized Gains 228   228   110
Level 2 securities, Gross Unrealized Losses (768)   (768)   (1,087)
Total, amortized cost 2,170,347   2,170,347   1,739,673
Total, estimated fair value 2,169,807   2,169,807   1,738,696
Fair Value, Measurements, Recurring | Level 1 Securities | Money market funds          
Schedule of Available-for-sale Securities [Line Items]          
Cash and Money Market Funds, Amortized Cost 275,248   275,248   204,967
Cash and Money Market Funds, Estimated Fair Value 273,978   273,978   203,450
Cash, Cash Equivalents and Restricted Cash, Noncurrent Assets 1,270   1,270   1,517
Fair Value, Measurements, Recurring | Level 1 Securities | Time deposits          
Schedule of Available-for-sale Securities [Line Items]          
Level 2 securities, Estimated Fair Value 300,000   300,000    
Fair Value, Measurements, Recurring | Level 2 Securities | Time deposits          
Schedule of Available-for-sale Securities [Line Items]          
Level 2 securities, Amortized Cost 300,000   300,000    
Level 2 securities, Estimated Fair Value 300,000   300,000    
Fair Value, Measurements, Recurring | Level 2 Securities | Corporate debt securities          
Schedule of Available-for-sale Securities [Line Items]          
Level 2 securities, Amortized Cost 189,190   189,190   199,843
Level 2 securities, Gross Unrealized Gains 228   228   110
Level 2 securities, Gross Unrealized Losses (392)   (392)   (731)
Level 2 securities, Estimated Fair Value 189,026   189,026   199,222
Fair Value, Measurements, Recurring | Level 2 Securities | Government securities          
Schedule of Available-for-sale Securities [Line Items]          
Level 2 securities, Amortized Cost 35,906   35,906   35,944
Level 2 securities, Gross Unrealized Losses (169)   (169)   (128)
Level 2 securities, Estimated Fair Value 35,737   35,737   35,816
Fair Value, Measurements, Recurring | Level 2 Securities | Certificates of deposit          
Schedule of Available-for-sale Securities [Line Items]          
Level 2 securities, Amortized Cost         9,833
Level 2 securities, Estimated Fair Value         9,833
Fair Value, Measurements, Recurring | Level 2 Securities | Agency securities          
Schedule of Available-for-sale Securities [Line Items]          
Level 2 securities, Amortized Cost 21,569   21,569   21,563
Level 2 securities, Gross Unrealized Losses (207)   (207)   (228)
Level 2 securities, Estimated Fair Value 21,362   21,362   21,335
Fair Value, Measurements, Recurring | Cash          
Schedule of Available-for-sale Securities [Line Items]          
Cash and Money Market Funds, Amortized Cost 1,348,434   1,348,434   1,267,523
Cash and Money Market Funds, Estimated Fair Value 1,344,799   1,344,799   1,264,126
Cash, Cash Equivalents and Restricted Cash, Noncurrent Assets 3,635   3,635   3,397
Cash and Cash Equivalents | Fair Value, Measurements, Recurring          
Schedule of Available-for-sale Securities [Line Items]          
Total, estimated fair value 1,918,777   1,918,777   1,467,576
Short-term Investments | Fair Value, Measurements, Recurring          
Schedule of Available-for-sale Securities [Line Items]          
Total, estimated fair value 246,125   246,125   266,206
Non-current Assets | Fair Value, Measurements, Recurring          
Schedule of Available-for-sale Securities [Line Items]          
Total, estimated fair value [1] $ 4,905   $ 4,905   $ 4,914
[1] Primarily restricted cash that is related to workers compensation deposits and letter of credit agreements.