Changes in Operations (Details 8)	3 Months Ended								12 Months Ended			1 Months Ended	12 Months Ended								12 Months Ended
	Dec. 31, 2011 USD ($)	Sep. 30, 2011 USD ($)	Jun. 30, 2011 USD ($)	Mar. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Sep. 30, 2010 USD ($)	Jun. 30, 2010 USD ($)	Mar. 31, 2010 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2010 FreightLink [Member] USD ($)	Dec. 31, 2009 FreightLink [Member] USD ($)	Dec. 01, 2010 FreightLink [Member] USD ($)	Sep. 30, 2010 FreightLink [Member] USD ($)	Jun. 30, 2010 FreightLink [Member] USD ($)	Mar. 31, 2010 FreightLink [Member] USD ($)	Sep. 30, 2009 FreightLink [Member] USD ($)	Jun. 30, 2009 FreightLink [Member] USD ($)	Mar. 31, 2009 FreightLink [Member] USD ($)	Dec. 31, 2011 AUSTRALIA	Dec. 31, 2010 GWA North [Member] USD ($)	Dec. 31, 2010 GWA North [Member] AUD	Dec. 31, 2009 GWA North [Member] USD ($)	Dec. 31, 2009 GWA North [Member] AUD
Statutory income tax rate									35.00%	35.00%	35.00%										30.00%
Revenues	$ 210,386,000	$ 217,210,000	$ 209,589,000	$ 191,911,000	$ 169,671,000	$ 156,492,000	$ 158,453,000	$ 145,579,000	$ 829,096,000	$ 630,195,000	$ 544,866,000											$ 11,600,000
Income from operations	45,388,000	56,023,000	51,165,000	39,203,000	23,919,000	38,512,000	37,873,000	30,106,000	191,779,000	130,410,000	99,322,000											10,300,000
FreightLink acquisition costs, net of tax												28,200,000	7,800,000
Stamp duty									$ 0	$ 12,625,000	$ 0											$ 11,500,000	11,800,000	$ 8,300,000	11,800,000
Foreign currency exchange rate, translation												0.99	0.91	0.96	0.91	0.88	0.90	0.83	0.76	0.66