| Significant Accounting Policies - Percentage of Revenue by Major Customers (Details) - Sales Revenue Net - Customer Concentration Risk | 12 Months Ended | ||
|---|---|---|---|
| Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | |
| Shell Trading (US) Co. | |||
| Entity Wide Revenue Major Customer [Line Items] | |||
| Percentage of receipts | 30.00% | 46.00% | 43.00% | 
| BP Products North America | |||
| Entity Wide Revenue Major Customer [Line Items] | |||
| Percentage of receipts | 20.00% | ||
| Vitol Inc. | |||
| Entity Wide Revenue Major Customer [Line Items] | |||
| Percentage of receipts | 14.00% | 15.00% | 20.00% | 
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- Definition 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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