Debt Obligations (Narrative) (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	May 06, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Long term obligations	$ 374,591,000		$ 384,515,000
Unsubordinated term loan and Revolving credit facility [Member]
Debt Instrument [Line Items]
Collateral coverage percentage	150.00%
Debt Instrument, Collateral, Coverage Percentage Total Revolver	175.00%
Debt Instrument, Collateral, Coverage Percentage Minimum	50.00%
Unsubordinated term loan [Member]
Debt Instrument [Line Items]
Long term obligations	127,500,000		131,250,000
Variable interest rate	2.54%
Revolving credit facility [Member]
Debt Instrument [Line Items]
Long term obligations	188,000,000		190,500,000
Accordion feature amount		50,000,000
Increased borrowing capacity	275,000,000
Variable interest rate	2.54%
Credit facility, revolving credit loan, remaining borrowing capacity	76,800,000
Letters of credit outstanding	10,200,000
Aircraft loans [Member]
Debt Instrument [Line Items]
Long term obligations	52,891,000		55,015,000
Collateralized property (in 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	6,200,000		7,750,000
Promissory note interest rate	5.00%
Prepayment of note per dollar of dividend distributed (in dollars per dividend)	0.20
Maximum [Member] | Unsubordinated term loan and Revolving credit facility [Member]
Debt Instrument [Line Items]
Maximum amount of common stock authorized for repurchase	$ 50,000,000