Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Jun. 30, 2012	Jun. 30, 2013 Recurring basis Level 2	Dec. 31, 2012 Recurring basis Level 2	Jun. 30, 2013 Nonrecurring basis Level 3	Jun. 30, 2012 Nonrecurring basis Level 3
Fair value measurements
Interest rate swaps asset		$ 132
Interest rate swaps liability		(3,276)	(9,916)
Impaired long-lived assets				$ 927	$ 7,436
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%