Acquisitions (Business Acquisition, Pro Forma Information) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Business Acquisition, Pro Forma Information [Abstract]
Business Acquisition, Pro Forma Information, Description	The summary below (in thousands, except for per share data) presents pro forma consolidated results of operations for the nine months ended September 30, 2012 and 2011 as if the acquisitions of HCP and Valesta occurred on January 1, 2010, and the acquisition of Apex occurred on January 1, 2011. The pro forma financial information gives effect to certain adjustments, including: the amortization of intangible assets and interest expense on acquisition-related debt, changes in the management fees, and increased number of common shares as a result of the acquisition. Acquisition-related costs are assumed to have occurred at the beginning of the year prior to acquisition. The pro-forma financial information is not necessarily indicative of the operating results that would have occurred if the acquisition had been consummated as of the date indicated, nor are they necessarily indicative of future operating results.
Revenues	$ 1,120,323	$ 961,320
Operating income	91,319	55,040
Net income	$ 42,506	$ 21,626
Basic earnings per share (in dollars per share)	$ 0.82	$ 0.42
Diluted earnings per share (in dollars per share)	$ 0.80	$ 0.42
Weighted average number of shares outstanding	51,929	51,170
Weighted average number of shares and dilutive shares outstanding	52,959	52,060