Business Segments, Concentration of Credit Risk and Significant Customers - Additional Information (Detail)	3 Months Ended	12 Months Ended
Mar. 31, 2019 Customer Segment	Dec. 31, 2018 Customer
Business Segments
Number of business segments \| Segment	1
Number of measurements of profitability	1
Accounts Receivable \| Major customers \| Credit Concentration
Business Segments
Percentage of concentration risk	94.00%	63.00%
Number of customer \| Customer	3	3

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

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Data Type:	num:percentItemType
Balance Type:	na
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