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, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Unsubordinated term loan and Revolving credit facility [Member]	Dec. 31, 2013 Unsubordinated term loan [Member]	Dec. 31, 2012 Unsubordinated term loan [Member]	Dec. 31, 2013 Revolving credit facility [Member]	Oct. 22, 2013 Revolving credit facility [Member]	Dec. 31, 2012 Revolving credit facility [Member]	Jul. 20, 2012 Revolving credit facility [Member]	Dec. 31, 2013 Aircraft loans [Member] aircrafts	Dec. 31, 2012 Aircraft loans [Member]	Dec. 31, 2013 Promissory note due to DHL, unsecured [Member]	Dec. 31, 2012 Promissory note due to DHL, unsecured [Member]	Dec. 31, 2013 Maximum [Member] Unsubordinated term loan and Revolving credit facility [Member]	Jan. 31, 2014 Subsequent Event [Member]
Debt Instrument [Line Items]
Long term obligations		$ 384,515,000	$ 364,481,000			$ 131,250,000	$ 144,375,000	$ 190,500,000		$ 143,000,000		$ 55,015,000	$ 63,156,000	$ 7,750,000	$ 13,950,000
Accordion feature amount									50,000,000		50,000,000
Increased borrowing capacity									275,000,000		225,000,000
Variable interest rate						2.55%		2.55%
Credit facility, revolving credit loan, remaining borrowing capacity								74,300,000
Letters of credit outstanding								10,200,000
Amount borrowed under line of credit																	15,000,000
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	$ 0	$ 2,886,000