Debt Obligations (Schedule of Long Term Obligations) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Total long term obligations	$ 410,073	$ 364,481
Less: current portion	(23,282)	(21,265)
Total long term obligations, net	386,791	343,216
Unsubordinated term loan [Member]
Debt Instrument [Line Items]
Total long term obligations	142,500	144,375
Revolving credit facility [Member]
Debt Instrument [Line Items]
Total long term obligations	194,000	143,000
Aircraft loans [Member]
Debt Instrument [Line Items]
Total long term obligations	61,173	63,156
Promissory note due to DHL, unsecured [Member]
Debt Instrument [Line Items]
Total long term obligations	$ 12,400	$ 13,950