Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounts receivable and other receivables
Reserves deducted from asset accounts:
Balance at Beginning of Year	$ 10,636	$ 0
Charged to Other Accounts		10,636
Balance at End of Year	10,636	10,636	$ 0
Current assets - supplies and inventory
Reserves deducted from asset accounts:
Balance at Beginning of Year	2,216	648	261
Additions (Reductions) Charged to Costs and Expenses	477	1,737	1,247
Charged to Other Accounts	(137)	(35)
Deductions	1,982	134	860
Balance at End of Year	574	2,216	648
Deferred income taxes
Reserves deducted from asset accounts:
Balance at Beginning of Year	506,316	530,612	610,571
Additions (Reductions) Charged to Costs and Expenses	76,524	(24,296)	(79,959)
Charged to Other Accounts	(8,845)
Balance at End of Year	$ 573,995	$ 506,316	$ 530,612