Significant Accounting Policies (Coal Supply Agreement) (Details)	12 Months Ended
Dec. 31, 2018
Maximum
Finite-Lived Intangible Assets [Line Items]
Coal supply agreement amortization period	30 years

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.

No definition available.

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration