Debt Obligations (Narrative) (Details) (USD $)	3 Months Ended	12 Months Ended						12 Months Ended			12 Months Ended		12 Months Ended		12 Months Ended
	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Unsubordinated term loan and Revolving credit facility [Member]	Dec. 31, 2012 Unsubordinated term loan [Member]	Dec. 31, 2011 Unsubordinated term loan [Member]	Dec. 31, 2012 Revolving credit facility [Member]	Jul. 20, 2012 Revolving credit facility [Member]	Dec. 31, 2011 Revolving credit facility [Member]	Dec. 31, 2012 Aircraft loans [Member] aircrafts	Dec. 31, 2011 Aircraft loans [Member]	Dec. 31, 2012 Promissory note due to DHL, unsecured [Member]	Dec. 31, 2011 Promissory note due to DHL, unsecured [Member]	Dec. 31, 2012 Maximum [Member] Unsubordinated term loan and Revolving credit facility [Member]	Jan. 31, 2013 Subsequent Event [Member]
Debt Instrument [Line Items]
Long term obligations		$ 364,481,000	$ 346,904,000			$ 144,375,000	$ 150,000,000	$ 143,000,000		$ 106,000,000	$ 63,156,000	$ 70,754,000	$ 13,950,000	$ 20,150,000
Accordion feature amount									50,000,000
Increased borrowing capacity									225,000,000
Variable interest rate						2.47%		2.47%
Credit facility, revolving credit loan, remaining borrowing capacity								70,900,000
Letters of credit outstanding								11,100,000
Amount borrowed under line of credit																60,000,000
Collateralized property (in aircrafts)											6
Balloon payment percentage											20.00%
Variable interest rate, minimum											6.74%
Variable interest rate, maximum											7.36%
Promissory note interest rate													5.00%
Collateral coverage percentage					150.00%
Maximum amount of common stock authorized for repurchase															50,000,000
Prepayment of note per dollar of dividend distributed (in dollars per dividend)													0.20
Write-off of unamortized debt issuance costs	$ 6,800,000	$ 0	$ 2,886,000	$ 0