Acquisitions (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Schedule of unaudited proforma information
Revenue - As reported	$ 1,563,860	$ 1,513,344	$ 3,100,136	$ 2,938,720
Net earnings - As reported	46,723	40,033	81,044	65,158
Net earnings per common share:
Basic - As reported	$ 0.92	$ 0.78	$ 1.59	$ 1.27
Diluted - As reported	$ 0.91	$ 0.77	$ 1.58	$ 1.26
Hill Hire [Member]
Schedule of unaudited proforma information
Revenue - Pro forma		1,543,084		3,006,290
Net earnings - Pro forma		$ 48,017		$ 80,230
Net earnings per common share:
Basic - Pro forma		$ 0.93		$ 1.56
Diluted - Pro forma		$ 0.93		$ 1.55