Acquisitions (Details 2) (USD $) In Thousands, except Per Share data, unless otherwise specified	2 Months Ended	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2012	Mar. 31, 2011
Supplemental pro forma results
Revenue	$ 397,455	$ 1,223,783	$ 855,246
Net income from continuing operations	$ 52,220	$ 110,648	$ 43,390
Basic earnings per share	$ 0.42	$ 0.71	$ 0.28
Diluted earnings per share	$ 0.41	$ 0.75	$ 0.28