Commitments and Contingencies - Concentrations of Credit Risk (Details) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2019 USD ($)	Mar. 31, 2018	Dec. 31, 2018
Vendor Reimbursement
Amount received from vendor	$ 3.5
Accounts receivable \| Credit risk \| Customers
Vendor Reimbursement
Concentration Risk Number of Customers	2		3
Concentration risk (as a percent)	83.20%		52.30%
Revenues \| Customer concentration
Vendor Reimbursement
Concentration Risk Number of Customers	2	2
Concentration risk (as a percent)	56.20%	74.20%

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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

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