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Summary of Significant Accounting Policies - Public Offering Costs (Details) - USD ($)
3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Accounting Policies [Abstract]      
Offering costs capitalized $ 0 $ 0 $ 0
Offering costs transferred to additional paid-in capital   $ 1,900,000  
Offering costs expensed $ 1,300,000   $ 1,300,000