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Cash and Investments (Summary by Balance Sheet Line Item) (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Investment [Line Items]    
Amortized Cost $ 380,438 $ 479,862
Gross Unrealized Gains 41 14
Gross Unrealized Losses (160) (322)
Fair Value 380,319 479,554
Cash and cash equivalents [Member]    
Investment [Line Items]    
Amortized Cost 135,212 151,686
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 135,212 151,686
Short-term investments [Member]    
Investment [Line Items]    
Amortized Cost 214,172 268,234
Gross Unrealized Gains 15 13
Gross Unrealized Losses (143) (130)
Fair Value 214,044 268,117
Long-term investments [Member]    
Investment [Line Items]    
Amortized Cost 26,404 55,792
Gross Unrealized Gains 26 1
Gross Unrealized Losses (17) (192)
Fair Value 26,413 55,601
Long-term restricted cash and investments [Member]    
Investment [Line Items]    
Amortized Cost 4,650 4,150
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value $ 4,650 $ 4,150