Debt Obligations (Narrative) (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Jul. 20, 2012
Debt Instrument [Line Items]
Long term obligations	$ 410,073,000	$ 364,481,000
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]
Variable interest rate	2.46%
Long term obligations	142,500,000	144,375,000
Revolving credit facility [Member]
Debt Instrument [Line Items]
Accordion feature amount			50,000,000
Increased borrowing capacity			225,000,000
Variable interest rate	2.46%
Credit facility, revolving credit loan, remaining borrowing capacity	19,800,000
Long term obligations	194,000,000	143,000,000
Letters of credit outstanding	11,200,000
Aircraft loans [Member]
Debt Instrument [Line Items]
Long term obligations	61,173,000	63,156,000
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	12,400,000	13,950,000
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