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Cash and Investments (Summary by Security Type) (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Investment [Line Items]    
Amortized Cost $ 457,174 $ 460,274
Gross Unrealized Gains 37 14
Gross Unrealized Losses (255) (322)
Fair Value 456,956 459,966
Money market funds [Member]    
Investment [Line Items]    
Amortized Cost 44,641 71,457
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 44,641 71,457
Commercial paper [Member]    
Investment [Line Items]    
Amortized Cost 193,968 165,375
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 193,968 165,375
Corporate bonds [Member]    
Investment [Line Items]    
Amortized Cost 175,244 152,712
Gross Unrealized Gains 37 3
Gross Unrealized Losses (220) (308)
Fair Value 175,061 152,407
U.S. Treasury and government sponsored enterprises [Member]    
Investment [Line Items]    
Amortized Cost 43,321 70,730
Gross Unrealized Gains 0 11
Gross Unrealized Losses (35) (14)
Fair Value $ 43,286 $ 70,727