Equipment Acquisitions, Dispositions and Depreciation and Impairment Policies (Deferred Gain Activity) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Movement in Deferred Gain Activity [Roll Forward]
Deferred Gains at Beginning of Period	$ 111,514	$ 117,192
Deferred Gain On Sale Of Equipments	2,289	7,280
Amortization Of Deferred Gains Included in Operating Expenses As A Reduction To Rental Expense	(5,192)	(9,526)
Amortization Of Deferred Gains Included In Gains On Asset Dispositions And Impairments, Net	(1,431)	(2,039)
Deferred Gains at End of Period	$ 107,180	$ 112,907