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Marketable Securities (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Amortized Cost    
Amortized cost $ 87,841 $ 147,370
Gross Unrealized Gains    
Gross Unrealized Gains 9 13
Gross Unrealized Losses    
Gross Unrealized Losses (28) (68)
Fair Value    
Fair Value 87,822 147,315
Marketable securities considered to be other-than-temporarily impaired 0  
Accrued interest included in marketable securities 400 600
U.S. government and municipal obligations    
Amortized Cost    
Maturing in one year or less 18,253 52,754
Maturing after one year through three years   296
Amortized cost 18,253 53,050
Gross Unrealized Gains    
Maturing in one year or less 7 5
Maturing after one year through three years   8
Gross Unrealized Gains 7 13
Gross Unrealized Losses    
Maturing in one year or less (6) (12)
Gross Unrealized Losses (6) (12)
Fair Value    
Maturing in one year or less 18,254 52,747
Maturing after one year through three years   304
Fair Value 18,254 53,051
Corporate debt securities    
Amortized Cost    
Maturing in one year or less 69,588 94,320
Amortized cost 69,588 94,320
Gross Unrealized Gains    
Maturing in one year or less 2  
Gross Unrealized Gains 2  
Gross Unrealized Losses    
Maturing in one year or less (22) (56)
Gross Unrealized Losses (22) (56)
Fair Value    
Maturing in one year or less 69,568 94,264
Fair Value $ 69,568 $ 94,264