Equipment Acquisitions, Dispositions and Depreciation and Impairment Policies (Deferred Gain Activity) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Movement in Deferred Gain Activity [Roll Forward]
Deferred Gains at Beginning of Period	$ 119,570	$ 131,836
Deferred Gain On Sale Of Equipments	7,280	4,597
Amortization Of Deferred Gains Included in Operating Expenses As A Reduction To Rental Expense	(9,526)	(11,194)
Amortization Of Deferred Gains Included In Gains On Asset Dispositions And Impairments, Net	(2,455)	(2,461)
Deferred Gains at End of Period	$ 114,869	$ 122,778