Acquisitions (Details 2) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Acquisition acquired activities
Revenue	$ 172,861	$ 183,800	$ 188,606	$ 220,827	$ 162,923	$ 177,349	$ 183,464	$ 210,069	$ 766,094	$ 733,805	$ 730,401
Revenue, Pro Forma Adjustments									48,808
Revenue, Pro Forma Consolidated									814,902
Net income	1,206	5,307	5,848	18,785	(1,166)	4,717	6,551	17,905	31,146	28,007	24,514
Net income, Pro Forma Adjustments									4,917
Net income, Pro Forma Consolidated									$ 36,063
Earnings per share:
Basic, Consolidated As Reported	$ 0.02	$ 0.11	$ 0.12	$ 0.38	$ (0.02)	$ 0.10	$ 0.13	$ 0.36	$ 0.63	$ 0.57	$ 0.42
Basic, Pro Forma Adjustments									$ 0.10
Basic, Pro Forma Consolidated									$ 0.73
Diluted, Consolidated As Reported	$ 0.02	$ 0.11	$ 0.12	$ 0.38	$ (0.02)	$ 0.10	$ 0.13	$ 0.36	$ 0.63	$ 0.56	$ 0.42
Diluted, Pro Forma Adjustments									$ 0.10
Diluted, Pro Forma Consolidated									$ 0.73
Weighted average common shares outstanding:
Basic weighted average common shares outstanding	48,967	48,895	49,041	49,103	48,854	49,525	49,615	49,322	49,002	49,328	57,692
Basic, Weighted average common shares outstanding Pro Forma Adjustments									508
Basic, Weighted average common shares outstanding Pro Forma Consolidated									49,510
Diluted weighted average common shares outstanding	49,326	49,109	49,244	49,531	48,854	49,920	49,958	49,755	49,252	49,599	58,193
Diluted, Weighted average common shares outstanding Pro Forma Adjustments									492
Diluted, Weighted average common shares outstanding Pro Forma Consolidated									49,744