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Cash and Investments (Available for Sale by Contractual Maturity) (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Mature within One Year $ 411,603  
After One Year through Two Years 45,353  
Fair Value 456,956 $ 459,966
Money market funds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Mature within One Year 44,641  
After One Year through Two Years 0  
Fair Value 44,641 71,457
Commercial paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Mature within One Year 193,968  
After One Year through Two Years 0  
Fair Value 193,968 165,375
Corporate bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Mature within One Year 132,355  
After One Year through Two Years 42,706  
Fair Value 175,061 152,407
U.S. Treasury and government sponsored enterprises [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Mature within One Year 40,639  
After One Year through Two Years 2,647  
Fair Value $ 43,286 $ 70,727