Summary of Significant Accounting Policies - Credit Risk (Details) - Customer concentration risk - Revenue	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
PEF
Concentration Risk [Line Items]
Percentage of revenue	9.00%
PEF Power Plant
Concentration Risk [Line Items]
Percentage of revenue		11.00%

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.

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