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
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13531-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13537-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Commitments and Contingencies - Concentrations of Credit Risk (Details) $ in Millions	3 Months Ended	6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($) customer	Jun. 30, 2019 USD ($) customer	Jun. 30, 2018	Dec. 31, 2018
Vendor Reimbursement
Amount received from vendor	$ 1.0	$ 4.5
Accounts receivable \| Credit risk \| Customers
Vendor Reimbursement
Concentration Risk Number of Customers	1	1		3
Concentration risk (as a percent)		71.00%		52.30%
Revenues \| Customer concentration
Vendor Reimbursement
Concentration Risk Number of Customers	2	2	2
Concentration risk (as a percent)		66.00%	66.20%