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Accounting Policies Intangible Assets (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Supply contracts
Acquired Finite-Lived Intangible Assets [Line Items]
Estimated useful life (years)	11 years 6 months	11 years 6 months

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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