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Marketable Securities - Continuous Unrealized Loss Position (Details)
$ in Thousands
Mar. 31, 2018
USD ($)
Fair Value  
Less than 12 months $ 14,202
12 months or greater 1,100
Unrealized Loss  
Less than 12 months (113)
12 months or greater (1)
U.S. treasury debt securities  
Fair Value  
Less than 12 months 1,980
12 months or greater 1,100
Unrealized Loss  
Less than 12 months (7)
12 months or greater (1)
U.S. corporate debt securities  
Fair Value  
Less than 12 months 12,222
12 months or greater 0
Unrealized Loss  
Less than 12 months (106)
12 months or greater $ 0