Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2012 Recurring basis Level 2	Dec. 31, 2011 Recurring basis Level 2	Dec. 31, 2012 Nonrecurring basis Level 3	Dec. 31, 2011 Nonrecurring basis Level 3
Fair value measurements
Interest rate swaps asset (liability)		$ (9,916)	$ (8,237)
Impaired long-lived assets				$ 7,527	$ 275
Period over which fleet assets are to be disposed of	12 months
Weighted average disposal period used in estimation of the fair value of the impaired long-lived assets	4 years
Discount rate used in estimation of the fair value of the impaired long-lived assets (as a percent)	10.40%