Fair Value Measurement (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2011
Financial liabilities measured at fair value on a recurring basis
Total liabilities at fair value	$ 23,698		$ 23,698		$ 20,907
Income approach valuation technique [Member]
Financial liabilities measured at fair value on a recurring basis
Contingent purchase price related to acquisitions	23,698		23,698		20,907
Level 1 [Member]
Financial liabilities measured at fair value on a recurring basis
Contingent purchase price related to acquisitions	0		0		0
Total liabilities at fair value
Level 2 [Member]
Financial liabilities measured at fair value on a recurring basis
Contingent purchase price related to acquisitions	0		0		0
Total liabilities at fair value	0		0		0
Level 3 [Member]
Financial liabilities measured at fair value on a recurring basis
Total liabilities at fair value	23,698		23,698		20,907
Balance, beginning of period	23,698	7,210	23,698	7,210	20,907	21,613	3,480	3,015
Earnouts and adjustments related to acquisitions	5,348	3,668	5,778	4,362	17,733
Payment of contingent purchase obligations	(2,407)	0	(2,407)	(284)	(284)
Adjustment to contingent purchase obligation	(856)	62	(580)	117	443
Balance, end of period	23,698	7,210	23,698	7,210	20,907	21,613	3,480	3,015
Level 3 [Member] | Income approach valuation technique [Member]
Financial liabilities measured at fair value on a recurring basis
Contingent purchase price related to acquisitions	$ 23,698		$ 23,698		$ 20,907