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Fair Value Measurement of Assets and Liabilities (Fair Value of Money Market Funds, Short and Long-Term Investments Classified as Available for Sale Securities) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 321,909 $ 125,843
Gross Unrealized Losses (6) (297) [1]
Estimated Fair Value 321,903 125,546
Cash and Cash Equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cost 272,258 85,337
Estimated Fair Value 272,258 85,337
Cash and Cash Equivalents [Member] | Money Market Funds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cost 272,258 85,337
Estimated Fair Value 272,258 85,337
Restricted Cash and Cash Equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cost 49,151  
Estimated Fair Value 49,151  
Restricted Cash and Cash Equivalents [Member] | Money Market Funds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cost 49,151  
Estimated Fair Value 49,151  
Short-term Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cost 61 6,631
Gross Unrealized Losses [1]   (19)
Estimated Fair Value 61 6,612
Short-term Investments [Member] | Sovereign Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cost 12 2,235
Gross Unrealized Losses [1]   (10)
Estimated Fair Value 12 2,225
Short-term Investments [Member] | Corporate Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cost 49 4,396
Gross Unrealized Losses [1]   (9)
Estimated Fair Value 49 4,387
Long-term Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cost 439 33,875
Gross Unrealized Losses (6) (278) [1]
Estimated Fair Value 433 33,597
Long-term Investments [Member] | Sovereign Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cost 190 13,821
Gross Unrealized Losses (2) (150) [1]
Estimated Fair Value 188 13,671
Long-term Investments [Member] | Corporate Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cost 249 20,054
Gross Unrealized Losses (4) (128) [1]
Estimated Fair Value $ 245 $ 19,926
[1] Unrealized gains (losses) from securities are attributable to market price movements, net foreign exchange losses and foreign currency translation. Management does not believe any remaining significant unrealized losses represent other-than-temporary impairments based on the evaluation of available evidence including the credit rating of the investments, as of September 30, 2018 and December 31, 2017.