Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8672-108599

Name:	us-gaap_NumberOfOperatingSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Nature of Operations and Basis of Presentation (Narrative) (Details) T in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021 operatingSegment T	Dec. 31, 2020 T	Dec. 31, 2019 T
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of operating segments \| operatingSegment	3
Coal Mining \| Bisti
Long-term Purchase Commitment [Line Items]
Historical lignite coal annual delivery		4.2	5.0
Minimum \| Coal Mining \| Bisti
Long-term Purchase Commitment [Line Items]
Expected future lignite coal annual delivery	3.3
Maximum \| Coal Mining \| Bisti
Long-term Purchase Commitment [Line Items]
Expected future lignite coal annual delivery	3.5