Debt Obligations (Schedule of Long Term Obligations) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Total long term obligations	$ 374,591	$ 384,515
Less: current portion	(23,873)	(23,721)
Total long term obligations, net	350,718	360,794
Unsubordinated term loan [Member]
Debt Instrument [Line Items]
Total long term obligations	127,500	131,250
Revolving credit facility [Member]
Debt Instrument [Line Items]
Total long term obligations	188,000	190,500
Aircraft loans [Member]
Debt Instrument [Line Items]
Total long term obligations	52,891	55,015
Promissory note due to DHL, unsecured [Member]
Debt Instrument [Line Items]
Total long term obligations	$ 6,200	$ 7,750