Debt Obligations (Narrative) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2015 USD ($) aircrafts	May. 05, 2015 USD ($)	Dec. 31, 2014 USD ($)
Debt Instrument [Line Items]
Long term obligations	$ 315,453		$ 344,094
Unsubordinated term loan and Revolving credit facility [Member]
Debt Instrument [Line Items]
Collateral coverage percentage	150.00%
Unsubordinated term loan [Member]
Debt Instrument [Line Items]
Long term obligations	$ 108,750		116,250
Increase in Additional Indebtedness. Long-Term Debt		$ 50,000
Additional Indebtedness Long-Term Debt		150,000
Variable interest rate	1.94%
Revolving credit facility [Member]
Debt Instrument [Line Items]
Long term obligations	$ 165,000		180,000
Accordion feature amount		50,000
Increased borrowing capacity		50,000
Line of Credit Facility, Maximum Borrowing Capacity		$ 325,000
Variable interest rate	1.94%
Credit facility, revolving credit loan, remaining borrowing capacity	$ 150,900
Letters of credit outstanding	9,100
Aircraft loans [Member]
Debt Instrument [Line Items]
Long term obligations	$ 41,703		46,294
Collateralized property (in aircrafts) | aircrafts	6
Balloon payment percentage	20.00%
Variable interest rate, minimum	6.74%
Variable interest rate, maximum	7.36%
Promissory note due to DHL, unsecured [Member]
Debt Instrument [Line Items]
Long term obligations	$ 0		$ 1,550
Maximum [Member] | Unsubordinated term loan and Revolving credit facility [Member]
Debt Instrument [Line Items]
Collateral coverage percentage	175.00%
Maximum amount of common stock authorized for repurchase	$ 50,000
Minimum [Member] | Unsubordinated term loan and Revolving credit facility [Member]
Debt Instrument [Line Items]
Collateral coverage percentage	50.00%