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Fair Value of Financial Instruments (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities $ 257,570 $ 261,208
Money market funds    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 11,078 10,915
US Treasury and Government Short-term Debt Securities [Member]    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 68,898 55,220
Corporate notes, bonds and commercial paper    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 177,594 195,073
Recurring basis    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 257,570 261,208
Recurring basis | Money market funds    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 11,078 10,915
Recurring basis | US Treasury and Government Short-term Debt Securities [Member]    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 68,898 55,220
Recurring basis | Corporate notes, bonds and commercial paper    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 177,594 195,073
Recurring basis | Quoted Market Prices in Active Markets (Level 1)    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 11,078 11,973
Recurring basis | Quoted Market Prices in Active Markets (Level 1) | Money market funds    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 11,078 10,915
Recurring basis | Quoted Market Prices in Active Markets (Level 1) | US Treasury and Government Short-term Debt Securities [Member]    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 0 0
Recurring basis | Quoted Market Prices in Active Markets (Level 1) | Corporate notes, bonds and commercial paper    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 0 1,058
Recurring basis | Significant Other Observable Inputs (Level 2)    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 246,492 249,235
Recurring basis | Significant Other Observable Inputs (Level 2) | Money market funds    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 0 0
Recurring basis | Significant Other Observable Inputs (Level 2) | US Treasury and Government Short-term Debt Securities [Member]    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 68,898 55,220
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate notes, bonds and commercial paper    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 177,594 194,015
Recurring basis | Significant Unobservable Inputs (Level 3)    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 0 0
Recurring basis | Significant Unobservable Inputs (Level 3) | Money market funds    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 0 0
Recurring basis | Significant Unobservable Inputs (Level 3) | US Treasury and Government Short-term Debt Securities [Member]    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities 0 0
Recurring basis | Significant Unobservable Inputs (Level 3) | Corporate notes, bonds and commercial paper    
Financial assets subject to fair value measurements and the necessary disclosures    
Total available-for-sale securities $ 0 $ 0