Debt Obligations (Details) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Debt Instrument [Line Items]
Total long term obligations	$ 363,195,000			$ 363,195,000		$ 346,904,000
Less: current portion	(17,239,000)			(17,239,000)		(13,223,000)
Total long term obligations, net	345,956,000			345,956,000		333,681,000
Write-off of unamortized debt issuance costs	0	16,000	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	148,125,000			148,125,000		150,000,000
Credit facility, term loan face amount	150,000,000			150,000,000
Variable interest rate	2.72%
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	2.50%
Credit facility, revolving credit loan, remaining borrowing capacity	24,800,000			24,800,000
Letters of credit outstanding	19,200,000			19,200,000
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	67,020,000			67,020,000		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	17,050,000			17,050,000		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