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Marketable Securities - Continuous Unrealized Loss Position (Details)
$ in Thousands
Dec. 31, 2017
USD ($)
Fair Value:  
Less than 12 months $ 15,265
12 months or greater 1,099
Unrealized Loss:  
Less than 12 months (68)
12 months or greater (1)
U.S. treasury debt securities  
Fair Value:  
Less than 12 months 1,976
12 months or greater 1,099
Unrealized Loss:  
Less than 12 months (7)
12 months or greater (1)
U.S. corporate debt securities  
Fair Value:  
Less than 12 months 13,289
12 months or greater 0
Unrealized Loss:  
Less than 12 months (61)
12 months or greater $ 0