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Short-term Investments (Available-For-Sale Securities Reported At Fair Value) (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Level 1 Securities [Member] | Money market funds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cash and Money market funds, Amortized Cost $ 204,967 $ 107,199
Cash and Money market funds, Fair Value Disclosure 204,967 107,199
Cash [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cash and Money market funds, Amortized Cost 1,267,523 1,708,220
Cash and Money market funds, Fair Value Disclosure 1,267,523 1,708,220
Fair Value, Measurements, Recurring [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Gross Unrealized Gains 110 154
Available-for-sale Securities, Gross Unrealized Losses (1,087) (1,464)
Total, Amortized Cost 1,739,673 2,318,114
Total, Estimated Fair Value 1,738,696 2,316,804
Fair Value, Measurements, Recurring [Member] | Cash and Cash Equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total, Estimated Fair Value 1,467,576 1,809,330
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total, Estimated Fair Value 266,206 501,385
Fair Value, Measurements, Recurring [Member] | Non-current Assets [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total, Estimated Fair Value [1] 4,914 6,089
Fair Value, Measurements, Recurring [Member] | Level 1 Securities [Member] | Money market funds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cash, Cash Equivalents and Restricted Cash, Gross Unrealized Gains 0 0
Cash, Cash Equivalents and Restricted Cash, Gross Unrealized Losses 0 0
Cash and Money market funds, Fair Value Disclosure 203,450 102,738
Cash and Money market funds, Non-current assets [1] 1,517 4,461
Fair Value, Measurements, Recurring [Member] | Level 2 Securities [Member] | Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Amortized Cost 199,843 240,867
Available-for-sale Securities, Gross Unrealized Gains 110 154
Available-for-sale Securities, Gross Unrealized Losses (731) (409)
Available-for-sale Securities, Estimated Fair Value 199,222 240,612
Fair Value, Measurements, Recurring [Member] | Level 2 Securities [Member] | Government securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Amortized Cost 35,944 235,252
Available-for-sale Securities, Gross Unrealized Gains 0 0
Available-for-sale Securities, Gross Unrealized Losses (128) (1,046)
Available-for-sale Securities, Estimated Fair Value 35,816 234,206
Fair Value, Measurements, Recurring [Member] | Level 2 Securities [Member] | Certificates of Deposit [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Amortized Cost 9,833  
Available-for-sale Securities, Gross Unrealized Gains 0  
Available-for-sale Securities, Gross Unrealized Losses 0  
Available-for-sale Securities, Estimated Fair Value 9,833  
Fair Value, Measurements, Recurring [Member] | Level 2 Securities [Member] | Agency securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Amortized Cost 21,563 26,576
Available-for-sale Securities, Gross Unrealized Gains 0 0
Available-for-sale Securities, Gross Unrealized Losses (228) (9)
Available-for-sale Securities, Estimated Fair Value 21,335 26,567
Fair Value, Measurements, Recurring [Member] | Cash [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cash, Cash Equivalents and Restricted Cash, Gross Unrealized Gains 0 0
Cash, Cash Equivalents and Restricted Cash, Gross Unrealized Losses 0 0
Cash and Money market funds, Fair Value Disclosure 1,264,126 1,706,592
Cash and Money market funds, Non-current assets [1] $ 3,397 $ 1,628
[1] Primarily restricted cash that is related to workers compensation deposits and letter of credit agreements.