Nature of Operations and Basis of Presentation (Narrative) (Details) $ in Thousands, T in Millions	3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) T	Sep. 30, 2019	Sep. 30, 2020 USD ($) segment	Dec. 31, 2019 USD ($) T	Dec. 31, 2018 T	Jul. 01, 2019 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of operating segments | segment			3
Long-term Purchase Commitment [Line Items]
Contractual obligation amount included in other noncurrent assets | $	$ 39,108		$ 39,108	$ 32,133
Coal Mining
Long-term Purchase Commitment [Line Items]
Historical lignite coal annual delivery				34.6
Outstanding securitized accounts receivable | $	2,300		$ 2,300
Coal Mining | Caddo Creek
Long-term Purchase Commitment [Line Items]
Historical lignite coal annual delivery				0.2
Coal Mining | Sabine
Long-term Purchase Commitment [Line Items]
Historical lignite coal annual delivery				2.6	3.8
Decrease in coal delivered			43.00%
NAMining | Lithium Nevada Corp.
Long-term Purchase Commitment [Line Items]
Term of contractual obligation		20 years
Contractual obligation amount provided | $						$ 3,500
Contractual obligation amount included in other noncurrent assets | $	$ 3,000		$ 3,000	$ 1,500
Minimum | Coal Mining | Sabine
Long-term Purchase Commitment [Line Items]
Historical lignite coal annual delivery				2.5
Expected future lignite coal annual delivery	1.4
Maximum | Coal Mining | Sabine
Long-term Purchase Commitment [Line Items]
Historical lignite coal annual delivery				3.5
Expected future lignite coal annual delivery	1.7