Debt Obligations (Schedule of Long Term Obligations) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Total long term obligations	$ 352,130	$ 384,515
Less: current portion	(24,027)	(23,721)
Total long term obligations, net	328,103	360,794
Unsubordinated term loan [Member]
Debt Instrument [Line Items]
Total long term obligations	123,750	131,250
Revolving credit facility [Member]
Debt Instrument [Line Items]
Total long term obligations	173,000	190,500
Aircraft loans [Member]
Debt Instrument [Line Items]
Total long term obligations	50,730	55,015
Promissory note due to DHL, unsecured [Member]
Debt Instrument [Line Items]
Total long term obligations	$ 4,650	$ 7,750