Debt Obligations (Details) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Mar. 31, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Debt Instrument [Line Items]
Total long term obligations	$ 357,855,000			$ 357,855,000		$ 346,904,000
Less: current portion	(19,251,000)			(19,251,000)		(13,223,000)
Total long term obligations, net	338,604,000			338,604,000		333,681,000
Write-off of unamortized debt issuance costs	0	0	6,800,000	0	2,886,000
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]
Total long term obligations	146,250,000			146,250,000		150,000,000
Credit facility, term loan face amount	150,000,000			150,000,000
Variable interest rate	2.62%
Revolving credit facility [Member]
Debt Instrument [Line Items]
Total long term obligations	131,000,000			131,000,000		106,000,000
Credit facility, revolving credit loan, maximum borrowing capacity	175,000,000			175,000,000
Variable interest rate	2.47%
Credit facility, revolving credit loan, remaining borrowing capacity	83,500,000			83,500,000
Letters of credit outstanding	10,500,000			10,500,000
Accordion feature amount	50,000,000			50,000,000
Increased borrowing capacity	225,000,000			225,000,000
Aircraft loans [Member]
Debt Instrument [Line Items]
Total long term obligations	65,105,000			65,105,000		70,754,000
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]
Total long term obligations	15,500,000			15,500,000		20,150,000
Promissory note interest rate	5.00%			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