Principles of Consolidation and Nature of Operations (Narrative) (Details) $ in Thousands, T in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) segment T	Dec. 31, 2019 USD ($) T
Long-term Purchase Commitment [Line Items]
Number of operating segments | segment	3
Earnings from unconsolidated operations | $	$ 60,203	$ 63,883
Coal Mining customer | Sabine
Long-term Purchase Commitment [Line Items]
Historical lignite coal annual delivery	1.9	2.6
Earnings from unconsolidated operations | $	$ 3,900	$ 4,600
Coal Mining customer | Sabine | Minimum
Long-term Purchase Commitment [Line Items]
Expected future lignite coal annual delivery	1.4
Coal Mining customer | Sabine | Maximum
Long-term Purchase Commitment [Line Items]
Expected future lignite coal annual delivery	1.7
Coal Mining customer | Caddo Creek
Long-term Purchase Commitment [Line Items]
Historical lignite coal annual delivery	0.1	0.2
Coal Mining customer | Camino Real Fuels, LLC
Long-term Purchase Commitment [Line Items]
Historical lignite coal annual delivery	0.3	1.6
Coal Mining customer | Falkirk
Long-term Purchase Commitment [Line Items]
Historical lignite coal annual delivery	7.2	7.4
Earnings from unconsolidated operations | $	$ 16,100	$ 15,900
Coal Mining customer | Falkirk | Long-term Contract with Customer
Long-term Purchase Commitment [Line Items]
Historical lignite coal annual delivery	0.3