Commitments and Contingencies - Concentrations of Credit Risk (Details)	9 Months Ended	12 Months Ended
Sep. 30, 2019 USD ($)	Sep. 30, 2018	Dec. 31, 2018
Vendor Reimbursement
Amount received from vendor	$ 4,500,000
Accounts receivable \| Credit risk \| Customers
Vendor Reimbursement
Concentration Risk Number of Customers	2		3
Concentration risk (as a percent)	74.20%		52.30%
Revenues \| Customer concentration
Vendor Reimbursement
Concentration Risk Number of Customers	3	2
Concentration risk (as a percent)	62.40%	66.30%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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