Debt Obligations (Schedule of Long Term Obligations) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Total long term obligations	$ 391,903	$ 364,481
Less: current portion	(23,572)	(21,265)
Total long term obligations, net	368,331	343,216
Unsubordinated term loan [Member]
Debt Instrument [Line Items]
Total long term obligations	135,000	144,375
Revolving credit facility [Member]
Debt Instrument [Line Items]
Total long term obligations	190,500	143,000
Aircraft loans [Member]
Debt Instrument [Line Items]
Total long term obligations	57,103	63,156
Promissory note due to DHL, unsecured [Member]
Debt Instrument [Line Items]
Total long term obligations	$ 9,300	$ 13,950