Nature Of Operations And Accounting Policies (Schedule Of Deferred Gain Activity) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2019
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Amortization of deferred gains included in operating expenses as a reduction to rental expense	$ 0	$ 0	$ 12,774
Operating Expense
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Amortization of deferred gains included in operating expenses as a reduction to rental expense	0	0	12,774
Gains on Asset Dispositions and Impairments, Net
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Amortization of deferred gains included in gains on asset dispositions, net	0	(1,127)	(11,591)
Amortization of deferred gains included in gains on asset dispositions, net	(1,322)	(1,322)	(1,372)
Sale Leaseback and Financed Equipment Sales
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Balance at beginning of year	8,768	39,102	66,519
Reclassification of deferred gains into historical cost on reacquired property and equipment	0	0	(3,052)
Balance at end of year	8,768	8,768	39,102
Fifty Percent or Less Owned Subsidiaries
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Balance at beginning of year	3,240	4,562	5,934
Balance at end of year	1,918	$ 3,240	$ 4,562
Sale Leaseback | Retained Earnings | Accounting Standards Update 2016-02
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Impact of adoption of accounting principle	$ 0			$ 29,207