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Cash and Investments (Summary by Balance Sheet Line Item) (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Investment [Line Items]    
Amortized Cost $ 475,992 $ 479,862
Gross Unrealized Gains 37 14
Gross Unrealized Losses (255) (322)
Fair Value 475,774 479,554
Cash and cash equivalents [Member]    
Investment [Line Items]    
Amortized Cost 183,179 151,686
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 183,179 151,686
Short-term investments [Member]    
Investment [Line Items]    
Amortized Cost 241,247 268,234
Gross Unrealized Gains 22 13
Gross Unrealized Losses (175) (130)
Fair Value 241,094 268,117
Long-term investments [Member]    
Investment [Line Items]    
Amortized Cost 47,416 55,792
Gross Unrealized Gains 15 1
Gross Unrealized Losses (80) (192)
Fair Value 47,351 55,601
Long-term restricted cash and investments [Member]    
Investment [Line Items]    
Amortized Cost 4,150 4,150
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value $ 4,150 $ 4,150