Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies - Contract Liabilities (Details)

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Summary of Significant Accounting Policies - Contract Liabilities (Details)
6 Months Ended
Jun. 30, 2019
USD ($)
Summary of Significant Accounting Policies  
Amounts included in contract liability at the beginning of the period $ 2,686,000