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5. Fair Value Measurements (Tables)
6 Months Ended
Jun. 30, 2018
Fair Value Measurements Tables Abstract  
Assets and liabilities measured at fair value on a recurring basis
  Level 1 Level 2 Level 3 Total
Assets        
Securities owned        
Municipal obligations 326,063 326,063
Corporate obligations 2,773 2,773
Total     $             328,836     $             328,836